The issue with the broken auto-generated avatars is exacerbated in Chrome.
There is no way to see who last posted in some of the threads shown in the screenshot above, unless you open the thread in question.
Unlike Firefox, Chrome doesn’t show placeholder images for the 404s, and collapses them to 0x0 pixel images, which means you can’t click on the avatar to bring up the user card, in order to see who posted. Nor can you hover the mouse pointer over the avatar area to see the tooltip showing the user’s (full or alt) name. As a result, there is no way to see who last posted in a given thread from the topic list.