You need to verify your account first. To do that, go to your email inbox and search for message from MPGH - Multiplayer Game Hacking. Make sure to check all spam and junk folders as well if you can't find it.
Then simply go to this page and change the avatar (the maximum size of your avatar is currently 150 x 175 pixles): https://www.mpgh.net/forum/profile.php?do=editavatar.