- last post: 01.01.0001 12:00 AM PDT
You can't, unless you have "teh skillz".
Basically, Bungie have disallowed all custom images on the Forums, to prevent vulgarity.
But -- custom avatars are allowed -- if you are a Bungie employee, or endure the harsh selection process to become one of their minions (a moderator).
Or, you can create your own Seventh Column theme -- as seen here.
And don't ask to become a moderator, if Bungie want you as one, then they'll give you the privileges.