Hello there CyberWolf!
Once you've updated your avatar on your main-site profile (which I see you have done), you will need to log out of both the main site and the forum and back in due to the way the both communicate with each other.
Try clearing your Cookies, Cache and Active Login data, selecting "Everything" for the amount of time from when to clear (see below image)
Simply log back in to the main site, then into the forum. Your forum avatar should have automatically updated by this point.
Please post back here if you're still having issues.
(Image: Clear Cookies, Cache, Active Logins [Firefox])