How do I add stuff to my profile? Banners, etc....

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I view other people's profiles and see all their nifty banners and ads for stuff and I can't figure out for the life of me how to do it myself! Maybe I'm just not recognizing it or maybe I'm just not looking in the right place. Heck! It took me over a week just to figure out how WHAT an Avatar was, let alone change mine! lol. Any help would be great. Thanks in advance!