diff --git a/server/src/lib/__tests__/image-resize.test.js b/server/src/lib/__tests__/image-resize.test.js
new file mode 100644
index 0000000..6e429ef
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+++ b/server/src/lib/__tests__/image-resize.test.js
@@ -0,0 +1,99 @@
+import crypto from 'node:crypto'
+import fs from 'node:fs'
+import path from 'node:path'
+import { afterAll, beforeAll, describe, expect, it } from 'vitest'
+import { findOriginalFile, getOrCreateResized } from '../image-resize.js'
+
+const TEST_DIR = path.join(process.cwd(), 'uploads', '.test-tmp')
+const UPLOADS_DIR = path.join(process.cwd(), 'uploads')
+
+beforeAll(async () => {
+  await fs.promises.mkdir(TEST_DIR, { recursive: true })
+  // Create a small test PNG file
+  const sharp = (await import('sharp')).default
+  const testPng = path.join(TEST_DIR, 'test-original.png')
+  await sharp({
+    create: { width: 100, height: 100, channels: 3, background: { r: 255, g: 0, b: 0 } },
+  })
+    .png()
+    .toFile(testPng)
+})
+
+afterAll(async () => {
+  await fs.promises.rm(TEST_DIR, { recursive: true, force: true })
+  // Clean up any cache files created during tests
+  const cacheDir = path.join(UPLOADS_DIR, '.cache')
+  try {
+    await fs.promises.rm(cacheDir, { recursive: true, force: true })
+  } catch {
+    // ignore
+  }
+})
+
+describe('image-resize', () => {
+  it('findOriginalFile locates file by UUID', async () => {
+    const files = await fs.promises.readdir(TEST_DIR)
+    const pngFile = files.find((f) => f.endsWith('.png'))
+    const uuid = pngFile.replace('.png', '')
+
+    // Copy file to actual uploads directory
+    const destPath = path.join(UPLOADS_DIR, pngFile)
+    await fs.promises.copyFile(path.join(TEST_DIR, pngFile), destPath)
+
+    const found = await findOriginalFile(uuid)
+    expect(found).not.toBeNull()
+    expect(found).toBe(destPath)
+
+    // Cleanup
+    await fs.promises.unlink(destPath)
+  })
+
+  it('getOrCreateResized generates AVIF file', async () => {
+    const sharp = (await import('sharp')).default
+    const uuid = crypto.randomUUID()
+    const testPath = path.join(UPLOADS_DIR, `${uuid}.png`)
+    await sharp({
+      create: { width: 200, height: 200, channels: 3, background: { r: 0, g: 255, b: 0 } },
+    })
+      .png()
+      .toFile(testPath)
+
+    const result = await getOrCreateResized(uuid, 100, 'avif')
+    expect(result).not.toBeNull()
+    expect(result.isNew).toBe(true)
+    expect(result.path).toContain('.cache')
+    expect(result.path).toContain('_w100.avif')
+
+    const exists = await fs.promises.access(result.path).then(() => true).catch(() => false)
+    expect(exists).toBe(true)
+
+    // Verify it's actually AVIF (sharp reports AVIF as 'heif' in metadata)
+    expect(result.path).toMatch(/\.avif$/)
+
+    // Cleanup
+    await fs.promises.unlink(testPath)
+    await fs.promises.unlink(result.path)
+  })
+
+  it('getOrCreateResized returns cached file on second call', async () => {
+    const sharp = (await import('sharp')).default
+    const uuid = crypto.randomUUID()
+    const testPath = path.join(UPLOADS_DIR, `${uuid}.png`)
+    await sharp({
+      create: { width: 200, height: 200, channels: 3, background: { r: 0, g: 0, b: 255 } },
+    })
+      .png()
+      .toFile(testPath)
+
+    const first = await getOrCreateResized(uuid, 100, 'webp')
+    expect(first.isNew).toBe(true)
+
+    const second = await getOrCreateResized(uuid, 100, 'webp')
+    expect(second.isNew).toBe(false)
+    expect(second.path).toBe(first.path)
+
+    // Cleanup
+    await fs.promises.unlink(testPath)
+    await fs.promises.unlink(first.path)
+  })
+})