Was just about to post a help topic on this too, here's the OP for that post:
ExampleFor me, when I go to a page on the forums a random selection of user's avatars will break and show the unknown image icon. It's not always consistent across users, as sometimes one person's avatar can be broken, but a refresh will solve it or vice versa. Sometimes none of the avatars on the page will show up at all!
There is one pattern I've noticed and that's that gif avatars tend to appear less often - this only happens to avatars, gifs in posts show up just fine.
Using Chrome 38 on Windows 8 (not 8.1).