"Weapons, hats, avatars -- designed by you, used by the entire TF2 community," explains Valve. "Pending approval, your item will get added to your backpack, and cycled into the random item drops in-game. Blow up stuff using a weapon you created! Kill an enemy, then let him know he died in a hat you designed!"
Alongside a submission form and the technical requirements for said items, the official "Contribute!" site packs item creation tips plus word that "Community-created avatars and items will eventually be displayed in an online image gallery."