diff --git a/.gitignore b/.gitignore
index 936e05f..702bbb5 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,5 +1,6 @@
 .claude/
 wiki/
+cache/
 
 # Binaries
 datascape
diff --git a/assets/diary/diary-day.html b/assets/diary/diary-day.html
index 05c4b84..7c069c6 100644
--- a/assets/diary/diary-day.html
+++ b/assets/diary/diary-day.html
@@ -1,7 +1,7 @@
 {{if .Photos}}
 <div class="photo-grid">
   {{range .Photos}}
-  <a href="{{.URL}}" target="_blank"><img src="{{.URL}}" alt="{{.Name}}" loading="lazy"></a>
+  <a href="{{.URL}}" target="_blank"><img src="{{.ThumbURL}}" alt="{{.Name}}" loading="lazy"></a>
   {{end}}
 </div>
 {{end}}
diff --git a/assets/diary/diary-month.html b/assets/diary/diary-month.html
index 7ef6988..74f5475 100644
--- a/assets/diary/diary-month.html
+++ b/assets/diary/diary-month.html
@@ -7,7 +7,7 @@
 {{if .Photos}}
 <div class="photo-grid">
   {{range .Photos}}
-  <a href="{{.URL}}" target="_blank"><img src="{{.URL}}" alt="{{.Name}}" loading="lazy"></a>
+  <a href="{{.URL}}" target="_blank"><img src="{{.ThumbURL}}" alt="{{.Name}}" loading="lazy"></a>
   {{end}}
 </div>
 {{end}}
diff --git a/assets/diary/diary-year.html b/assets/diary/diary-year.html
index 263677f..f7b609b 100644
--- a/assets/diary/diary-year.html
+++ b/assets/diary/diary-year.html
@@ -2,6 +2,12 @@
 {{range .Months}}
 <h3 id="{{.ID}}">
   <a href="{{.URL}}">{{.Name}}</a>
-  {{if .PhotoCount}}<span class="muted">({{.PhotoCount}} photos)</span>{{end}}
 </h3>
+{{if .Photos}}
+<div class="photo-grid">
+  {{range .Photos}}
+  <a href="{{.URL}}" target="_blank"><img src="{{.ThumbURL}}" alt="{{.Name}}" loading="lazy"></a>
+  {{end}}
+</div>
+{{end}}
 {{end}}
diff --git a/diary.go b/diary.go
index d307f1a..85f348a 100644
--- a/diary.go
+++ b/diary.go
@@ -283,16 +283,17 @@ func computeCalendarWidget(diaryRootFS, diaryRootURL, fsPath string, depth int)
 
 // diaryPhoto is a photo file whose name starts with a YYYY-MM-DD date prefix.
 type diaryPhoto struct {
-	Date time.Time
-	Name string
-	URL  string
+	Date     time.Time
+	Name     string
+	URL      string
+	ThumbURL string
 }
 
 type diaryMonthSummary struct {
-	ID         string
-	Name       string
-	URL        string
-	PhotoCount int
+	ID     string
+	Name   string
+	URL    string
+	Photos []diaryPhoto
 }
 
 type diaryDaySection struct {
@@ -376,10 +377,16 @@ func yearPhotos(yearFsPath, yearURLPath string) []diaryPhoto {
 		if err != nil {
 			continue
 		}
+		photoURL := path.Join(yearURLPath, url.PathEscape(name))
+		thumb := photoURL
+		if hasThumbnail(name) {
+			thumb = thumbURL(photoURL, 300)
+		}
 		photos = append(photos, diaryPhoto{
-			Date: t,
-			Name: name,
-			URL:  path.Join(yearURLPath, url.PathEscape(name)),
+			Date:     t,
+			Name:     name,
+			URL:      photoURL,
+			ThumbURL: thumb,
 		})
 	}
 	return photos
@@ -408,18 +415,18 @@ func renderDiaryYear(fsPath, urlPath string) template.HTML {
 		if err != nil || monthNum < 1 || monthNum > 12 {
 			continue
 		}
-		count := 0
+		var monthPhotos []diaryPhoto
 		for _, p := range photos {
 			if p.Date.Year() == year && int(p.Date.Month()) == monthNum {
-				count++
+				monthPhotos = append(monthPhotos, p)
 			}
 		}
 		monthDate := time.Date(year, time.Month(monthNum), 1, 0, 0, 0, 0, time.UTC)
 		months = append(months, diaryMonthSummary{
-			ID:         monthDate.Format("2006-01"),
-			Name:       monthDate.Format("January 2006"),
-			URL:        path.Join(urlPath, e.Name()) + "/",
-			PhotoCount: count,
+			ID:     monthDate.Format("2006-01"),
+			Name:   fmt.Sprintf("%s %d", germanMonths[monthDate.Month()], year),
+			URL:    path.Join(urlPath, e.Name()) + "/",
+			Photos: monthPhotos,
 		})
 	}
 
diff --git a/main.go b/main.go
index 1d408bf..d2b4268 100644
--- a/main.go
+++ b/main.go
@@ -41,6 +41,7 @@ var pageTypeHandlers []pageTypeHandler
 func main() {
 	addr := flag.String("addr", ":8080", "listen address")
 	wikiDir := flag.String("dir", "./wiki", "wiki root directory")
+	cacheDir := flag.String("cache", "./cache", "thumbnail cache directory")
 	user := flag.String("user", "", "basic auth username (empty = no auth)")
 	pass := flag.String("pass", "", "basic auth password")
 	flag.Parse()
@@ -53,6 +54,14 @@ func main() {
 		log.Fatal(err)
 	}
 
+	thumbCacheDir, err = filepath.Abs(*cacheDir)
+	if err != nil {
+		log.Fatal(err)
+	}
+	if err := os.MkdirAll(thumbCacheDir, 0755); err != nil {
+		log.Fatal(err)
+	}
+
 	initMarkdown(root)
 
 	authKey, err := loadOrCreateAuthKey(root)
@@ -86,6 +95,11 @@ func (h *handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	if strings.HasPrefix(r.URL.Path, thumbURLPrefix+"/") {
+		h.handleThumb(w, r)
+		return
+	}
+
 	urlPath := path.Clean("/" + r.URL.Path)
 	fsPath := filepath.Join(h.root, filepath.FromSlash(urlPath))
 
diff --git a/thumb.go b/thumb.go
new file mode 100644
index 0000000..2a06e58
--- /dev/null
+++ b/thumb.go
@@ -0,0 +1,224 @@
+package main
+
+import (
+	"bytes"
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+	"io"
+	"log"
+	"net/http"
+	"net/url"
+	"os"
+	"path"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"sync"
+	"time"
+)
+
+// Thumbnailer produces a thumbnail of a source file. Implementations register
+// themselves in init() by appending to thumbnailers. The first registered
+// handler whose CanHandle returns true is used.
+type Thumbnailer interface {
+	CanHandle(ext string) bool
+	Generate(src io.Reader, dst io.Writer, width int) error
+}
+
+var thumbnailers []Thumbnailer
+
+// thumbCacheDir is set from the -cache flag at startup.
+var thumbCacheDir string
+
+const thumbURLPrefix = "/_thumb"
+
+// Cache is content-addressed: the cache path is derived from the SHA-256 of
+// the source file. Renames and moves reuse the same cache entry; overwriting
+// a file with new content produces a new digest and regenerates.
+var (
+	thumbLocks   = map[string]*sync.Mutex{}
+	thumbLocksMu sync.Mutex
+
+	digestCache  = map[string]digestEntry{}
+	digestCacheMu sync.Mutex
+)
+
+// digestEntry remembers the digest of a source file so repeated requests do
+// not re-hash the whole file. The (mtime, size) pair invalidates the cache
+// when the file is overwritten in place.
+type digestEntry struct {
+	mtime time.Time
+	size  int64
+	hex   string
+}
+
+func findThumbnailer(name string) Thumbnailer {
+	ext := strings.ToLower(filepath.Ext(name))
+	if ext == "" {
+		return nil
+	}
+	for _, t := range thumbnailers {
+		if t.CanHandle(ext) {
+			return t
+		}
+	}
+	return nil
+}
+
+// hasThumbnail reports whether a file name has a registered thumbnailer.
+func hasThumbnail(name string) bool {
+	return findThumbnailer(name) != nil
+}
+
+// thumbURL builds a thumbnail URL for a wiki file. filePath must be URL-style
+// (slash-separated, leading slash), as already used on page links.
+func thumbURL(filePath string, width int) string {
+	return thumbURLPrefix + filePath + "?w=" + strconv.Itoa(width)
+}
+
+func (h *handler) handleThumb(w http.ResponseWriter, r *http.Request) {
+	raw := strings.TrimPrefix(r.URL.Path, thumbURLPrefix)
+	if raw == "" || raw == "/" {
+		http.NotFound(w, r)
+		return
+	}
+	decoded, err := url.PathUnescape(raw)
+	if err != nil {
+		http.Error(w, "bad path", http.StatusBadRequest)
+		return
+	}
+	cleanPath := path.Clean(decoded)
+
+	srcFS := filepath.Join(h.root, filepath.FromSlash(cleanPath))
+	rel, err := filepath.Rel(h.root, srcFS)
+	if err != nil || strings.HasPrefix(rel, "..") {
+		http.Error(w, "Forbidden", http.StatusForbidden)
+		return
+	}
+
+	srcInfo, err := os.Stat(srcFS)
+	if err != nil || srcInfo.IsDir() {
+		http.NotFound(w, r)
+		return
+	}
+
+	t := findThumbnailer(srcFS)
+	if t == nil {
+		http.NotFound(w, r)
+		return
+	}
+
+	width := 300
+	if s := r.URL.Query().Get("w"); s != "" {
+		if n, err := strconv.Atoi(s); err == nil && n > 0 && n <= 2000 {
+			width = n
+		}
+	}
+
+	digest, data, err := sourceDigest(srcFS, srcInfo)
+	if err != nil {
+		log.Printf("thumb digest %s: %v", rel, err)
+		http.Error(w, "thumbnail failed", http.StatusInternalServerError)
+		return
+	}
+
+	cacheFS := filepath.Join(thumbCacheDir, digest[:2], fmt.Sprintf("%s.%d.jpg", digest, width))
+	if _, err := os.Stat(cacheFS); err == nil {
+		serveThumb(w, r, cacheFS)
+		return
+	}
+
+	lock := thumbLock(cacheFS)
+	lock.Lock()
+	defer lock.Unlock()
+
+	if _, err := os.Stat(cacheFS); err == nil {
+		serveThumb(w, r, cacheFS)
+		return
+	}
+
+	var src io.Reader
+	if data != nil {
+		src = bytes.NewReader(data)
+	} else {
+		f, err := os.Open(srcFS)
+		if err != nil {
+			log.Printf("thumb open %s: %v", rel, err)
+			http.Error(w, "thumbnail failed", http.StatusInternalServerError)
+			return
+		}
+		defer f.Close()
+		src = f
+	}
+
+	if err := generateThumb(t, src, cacheFS, width); err != nil {
+		log.Printf("thumb %s: %v", rel, err)
+		http.Error(w, "thumbnail failed", http.StatusInternalServerError)
+		return
+	}
+	serveThumb(w, r, cacheFS)
+}
+
+// sourceDigest returns the SHA-256 hex digest of a source file's content.
+// On a cache hit (path + mtime + size unchanged) the returned data is nil,
+// so the caller knows to open the file itself. On a miss the file is read
+// once and the contents are returned for the caller to reuse.
+func sourceDigest(srcFS string, info os.FileInfo) (string, []byte, error) {
+	digestCacheMu.Lock()
+	d, ok := digestCache[srcFS]
+	digestCacheMu.Unlock()
+	if ok && d.mtime.Equal(info.ModTime()) && d.size == info.Size() {
+		return d.hex, nil, nil
+	}
+
+	data, err := os.ReadFile(srcFS)
+	if err != nil {
+		return "", nil, err
+	}
+	sum := sha256.Sum256(data)
+	h := hex.EncodeToString(sum[:])
+
+	digestCacheMu.Lock()
+	digestCache[srcFS] = digestEntry{mtime: info.ModTime(), size: info.Size(), hex: h}
+	digestCacheMu.Unlock()
+
+	return h, data, nil
+}
+
+func serveThumb(w http.ResponseWriter, r *http.Request, cacheFS string) {
+	w.Header().Set("Cache-Control", "public, max-age=86400")
+	http.ServeFile(w, r, cacheFS)
+}
+
+func generateThumb(t Thumbnailer, src io.Reader, cacheFS string, width int) error {
+	if err := os.MkdirAll(filepath.Dir(cacheFS), 0755); err != nil {
+		return err
+	}
+	tmp, err := os.CreateTemp(filepath.Dir(cacheFS), ".thumb-*")
+	if err != nil {
+		return err
+	}
+	tmpName := tmp.Name()
+	if err := t.Generate(src, tmp, width); err != nil {
+		tmp.Close()
+		os.Remove(tmpName)
+		return err
+	}
+	if err := tmp.Close(); err != nil {
+		os.Remove(tmpName)
+		return err
+	}
+	return os.Rename(tmpName, cacheFS)
+}
+
+func thumbLock(key string) *sync.Mutex {
+	thumbLocksMu.Lock()
+	defer thumbLocksMu.Unlock()
+	m, ok := thumbLocks[key]
+	if !ok {
+		m = &sync.Mutex{}
+		thumbLocks[key] = m
+	}
+	return m
+}
diff --git a/thumb_image.go b/thumb_image.go
new file mode 100644
index 0000000..aae2221
--- /dev/null
+++ b/thumb_image.go
@@ -0,0 +1,81 @@
+package main
+
+import (
+	"image"
+	"image/color"
+	_ "image/gif"
+	"image/jpeg"
+	_ "image/png"
+	"io"
+)
+
+func init() {
+	thumbnailers = append(thumbnailers, &imageThumbnailer{})
+}
+
+type imageThumbnailer struct{}
+
+func (it *imageThumbnailer) CanHandle(ext string) bool {
+	switch ext {
+	case ".jpg", ".jpeg", ".png", ".gif":
+		return true
+	}
+	return false
+}
+
+func (it *imageThumbnailer) Generate(src io.Reader, dst io.Writer, width int) error {
+	img, _, err := image.Decode(src)
+	if err != nil {
+		return err
+	}
+	return jpeg.Encode(dst, resizeBox(img, width), &jpeg.Options{Quality: 80})
+}
+
+// resizeBox downsamples src to the requested width using a box filter.
+// Aspect ratio is preserved. Upscaling is a no-op (returns src unchanged).
+// Each source pixel is visited exactly once; alpha is discarded.
+func resizeBox(src image.Image, width int) image.Image {
+	b := src.Bounds()
+	srcW, srcH := b.Dx(), b.Dy()
+	if srcW <= width {
+		return src
+	}
+	dstW := width
+	dstH := srcH * width / srcW
+	if dstH < 1 {
+		dstH = 1
+	}
+	dst := image.NewRGBA(image.Rect(0, 0, dstW, dstH))
+
+	for y := 0; y < dstH; y++ {
+		sy0 := y * srcH / dstH
+		sy1 := (y + 1) * srcH / dstH
+		if sy1 == sy0 {
+			sy1 = sy0 + 1
+		}
+		for x := 0; x < dstW; x++ {
+			sx0 := x * srcW / dstW
+			sx1 := (x + 1) * srcW / dstW
+			if sx1 == sx0 {
+				sx1 = sx0 + 1
+			}
+			var r, g, bl, n uint64
+			for sy := sy0; sy < sy1; sy++ {
+				for sx := sx0; sx < sx1; sx++ {
+					sr, sg, sb, _ := src.At(b.Min.X+sx, b.Min.Y+sy).RGBA()
+					r += uint64(sr >> 8)
+					g += uint64(sg >> 8)
+					bl += uint64(sb >> 8)
+					n++
+				}
+			}
+			dst.SetRGBA(x, y, color.RGBA{
+				R: uint8(r / n),
+				G: uint8(g / n),
+				B: uint8(bl / n),
+				A: 255,
+			})
+		}
+	}
+	return dst
+}