The dashboard now helps your browser cache the thumbnails in an intelligent, making scrolling through images more smooth:

You’ll notice that in the past scrolling through many images caused lots of flickering thumbnails: they were constantly being reloaded from our image servers, each time causing a brief moment where a white space was shown instead of the thumbnail, even if you just viewed them a few seconds ago.

Now when you scroll through images you’ve already seen, thumbnails will appear instantly from your browser cache.

The thumbnail could be missing if the image is very large, that’s also fixed.