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It displays once to reduce screen use. I'd prefer to display it each time, but I didn't want to expand the layout too much...

I'll try a background on the comment block. What window size are you using? If the window is narrower than about 800px the comments aren't placed in a separate column from the thumb. But normally (with a non-thumbnail-sized browser window), the avatars seem pretty distinct from the image thumbnails.

Avatars are shown in comments, forum posts and user profiles; they're not shown in posts (the focus there is the post).