I keep having this problem. Ever since I have come to CA, I'm not sure what to do about it, sometimes the image that is replacing a smiley is so large I have to block images to actually read posts. Sometimes reloading the page does it, but in today's example NOTHING has worked.
It seems that a random image from a thread I view, just suddenly out of nowhere replaces some random forum image. In this example it's the user_offline.gif, I have been looking around the forums and haven't seen anything on this bug.