@mecha (View Post)
Gravatar, when sending your avatar image for display in the forums, sends the image data along with a little note that says "hey, this avatar isn't something that changes often, so feel free to keep it offline for further use for at least a week or so!"

The time varies, of course, and I can't find a request to Gravatar to check, but the end result is that your browser remembers the old image and doesn't really bother asking for the new. Give it a couple of hours or days, or just tell your browser to reload and clear the cache for this site. In Chrome you can do it with Shift+F5.