Hmmm. It appears to me that the ManBoard has been hacked:
http://www.simplemachines.org/community/index.php?topic=307717.0
Many thanks to Mark for alerting me to this problem, and also for digging into the issue and coming up with some answers while I've been away. Definitely worth a pint next time we meet.
The bad news is that we had indeed been hacked again, along with what seems to be many other bulletin boards using the same underlying engine. The good news is that, as this is a well supported engine, the backroom boys had done a great job in creating some validation and correction scripts to confirm and then repair the damage. All applied now, as has the series of upgrades needed to remove the vulnerability.
Some avatars are still missing, but please feel free to re-instate yours now if desired (the hack was originally inserted through a rogue avatar). Let me know if it's still not displaying for you.
We're none of us ever safe from malicious attacks whilst online, so be very careful out there.