diff --git a/docs/openapi.yaml b/docs/openapi.yaml
index 929e36008..759e1f756 100644
--- a/docs/openapi.yaml
+++ b/docs/openapi.yaml
@@ -635,7 +635,12 @@ paths:
                   description: |
                     Avatar image file.
 
-                    Maximum size is 5 MiB.
+                    Avatar requirements:
+
+                    - The image format must be PNG (`image/png`) or JPEG (`image/jpeg`).
+                    - The maximum file size is 5 MiB.
+
+                    Uploaded avatars are normalized to a 512x512 PNG. Upload a square image for best visual quality.
                   type: string
                   format: binary
             encoding:
@@ -934,7 +939,12 @@ paths:
                   description: |
                     Avatar image file.
 
-                    Maximum size is 5 MiB.
+                    Avatar requirements:
+
+                    - The image format must be PNG (`image/png`) or JPEG (`image/jpeg`).
+                    - The maximum file size is 5 MiB.
+
+                    Uploaded avatars are normalized to a 512x512 PNG. Upload a square image for best visual quality.
                   type: string
                   format: binary
             encoding:
@@ -4832,7 +4842,12 @@ paths:
                   description: |
                     Avatar image file.
 
-                    Maximum size is 5 MiB.
+                    Avatar requirements:
+
+                    - The image format must be PNG (`image/png`) or JPEG (`image/jpeg`).
+                    - The maximum file size is 5 MiB.
+
+                    Uploaded avatars are normalized to a 512x512 PNG. Upload a square image for best visual quality.
                   type: string
                   format: binary
             encoding:
diff --git a/spx-gui/src/apps/xbuilder/pages/admin/user.vue b/spx-gui/src/apps/xbuilder/pages/admin/user.vue
index b377d7b3a..02a0f1fee 100644
--- a/spx-gui/src/apps/xbuilder/pages/admin/user.vue
+++ b/spx-gui/src/apps/xbuilder/pages/admin/user.vue
@@ -51,6 +51,10 @@ import * as authorizationAdminApis from '@/apis/admin/authorization'
 import { validateAccountUserPassword } from '@/components/account/admin/password'
 import { formatJSON, formatTime } from './common'
 
+const avatarSize = 512
+const maxAvatarFileSize = 5 * 1024 * 1024
+const acceptedAvatarContentTypes = new Set(['image/png', 'image/jpeg'])
+
 const props = defineProps<{
   userID: string
 }>()
@@ -255,12 +259,29 @@ async function handleAvatarFile(event: Event) {
   ;(event.target as HTMLInputElement).value = ''
 }
 
+function validateAvatarFile(file: File) {
+  if (file.type !== '' && !acceptedAvatarContentTypes.has(file.type)) {
+    throw new DefaultException({
+      en: 'Avatar image must be PNG or JPEG',
+      zh: '头像图片必须是 PNG 或 JPEG'
+    })
+  }
+
+  if (file.size > maxAvatarFileSize) {
+    throw new DefaultException({
+      en: 'Avatar image must be 5 MB or smaller',
+      zh: '头像图片不能超过 5 MB'
+    })
+  }
+}
+
 function hideBrokenImage(event: Event) {
   ;(event.currentTarget as HTMLImageElement).style.display = 'none'
 }
 
 const handleUpdateAvatar = useMessageHandle(
   async (file: File) => {
+    validateAvatarFile(file)
     await accountAdminApis.updateAccountUserAvatar(props.userID, file)
     userQuery.refetch()
   },
@@ -444,7 +465,14 @@ function deleteAllSessions() {
                   }}
                   <input class="hidden" type="file" accept="image/png,image/jpeg" @change="handleAvatarFile" />
                 </label>
-                <div class="text-center text-xs text-grey-700">PNG / JPG, ≤ 5 MB</div>
+                <div class="text-center text-xs text-grey-700">
+                  {{
+                    $t({
+                      en: `PNG or JPG, up to 5 MB. Saved as ${avatarSize}x${avatarSize} PNG.`,
+                      zh: `支持 PNG 或 JPG，最大 5 MB。保存为 ${avatarSize}x${avatarSize} PNG。`
+                    })
+                  }}
+                </div>
               </div>
               <form class="flex min-w-0 flex-col gap-5" @submit.prevent="handleUpdateUser.fn">
                 <div class="grid grid-cols-1 gap-4 tablet:grid-cols-2">