I often find image posts where clicking on the thumbnail doesn't load the picture and shows a generic error message. If I copy the image link, I can open it ok in the browser. I haven't been able to tie this to specific instances or communities, so I don't know what might be causing it.
Not sure if it's exactly the same issue, but I have noticed some images not loading, instead having a small "OBJ" rectangle in their place. Waiting or refreshing the page sometimes fixes it, sometimes doesn't.