On the right-hand panel, under the Manage header, there's a "Character" link. Once that page loads, toward the top of the page there'll be a "Edit Valk Dragonbane's Profile" link. This takes you to the page where you can edit things such as your description, forum signature, and avatar. The avatar is the "character picture", and you just put in the URL to an avatar that you like. It's most polite to not stream it from someone else's website, so you'll want to go to imageshack.us to host it. There you can just upload it, and it'll give you the URL to the newly hosted image.
May the Grace of Ara go with you, and may His Vengeance be wrought upon your enemies.