What is the broken image's path? Is it trying to use the gravatar url? If you disable looking for gravatars then does it display correctly?. You might like to open a ticket on open.silverstripe.com to track this issue.
I posted a possible fix at the link above.
On the ticket, I meant to say line 250 on ForumRole.php. I don't claim to know what I'm doing, so someone should verify the solution works and is the best solution. :-)