diff --git a/ui/component/selectThumbnail/view.jsx b/ui/component/selectThumbnail/view.jsx
index a96fc11a0..dcda827f2 100644
--- a/ui/component/selectThumbnail/view.jsx
+++ b/ui/component/selectThumbnail/view.jsx
@@ -35,7 +35,7 @@ class SelectThumbnail extends React.PureComponent<Props> {
 
     updatePublishForm({
       thumbnail: newThumbnail,
-      thumbnailError: false,
+      thumbnailError: newThumbnail.startsWith('data:image'),
     });
   }