I’m going to put three(!) ideas in here that are quick. The Twitter API has now given access to your profile background image (update_profile_background_image), profile avatar (update_profile_image), and profile design colors (update_profile_colors). Apparently Ian Schafer already attempted doing this on a one-off basis via eBay, but it looks like eBay pulled the auction.
The advertiser side of the equation: They will want to go through a directory of users, likely categorized by # of followers, and/or by the geo-location of the user’s followers (which can be found-out via the API), and/or by keyword phrases the user tends to mention in their tweets.
The publisher (typical user that’s renting their bg/avatar) side of the equation: the user would login with their Twitter credentials and simply approve (or enable auto-approve) of ads. And/or notifications are sent to user by email that an advertiser wishes to rent their space and then easily displays to the user whether they approve for the specific amount.
Payment by PayPal would be efficient.
Unfortunately, the Twitter API doesn’t allow for easy updating of a user’s bio or user’s URL — so there’s no real way to link back to the advertiser, unless the latest tweet referred to the advertiser’s promo.
Bonus Idea #2: This is a lot simpler, but requires TwitPic to do it, or someone to duplicate TwitPic. When I send in a photo from my cell phone via TwitPic, why not allow me to send in the photo to a different email address (or the same, but with a ‘#bg’ in the title of the photo sent in) and then TwitPic makes that my new background image for my profile. Could do the same with ‘#av’ for my avatar.
Bonus Idea #3: With the API, you could create a simple 1-page website that allows a Twitter user to easily switch-out their background image or profile avatar, with 1-click. If my Twitter login/pw is stored with your website, then I could easily click on a different profile avatar image (that I uploaded, and/or that you offered up) or background image. My thought: what about a simple designation of ‘offline’ and ‘online’? Basically, you could grab my current avatar, and then create a version where the text ‘online’ is on it somewhere [show me like 4 variations and I'll pick whether I want that text in upper/lower left/right corner] and ‘offline’ is on it somewhere. Maybe ‘offline’ image is now in grayscale.
The one problem with this online/offline thing is — who is going to change their routine to click ‘offline’ before signing-off for the day?
There could be other images for holidays — this could pull your background image and apply a Halloween theme to it; or snow flakes (for winter). Or maybe it checks the current weather in your city/state/country and posts it right in the image itself for the day (or through-out the day, updating it).
It may also tweet out for the user that their profile or bg image was changed using (your service name) — so that you can get some awareness/credit and more users. [and/or post in the header/footer of the user's background image]
You may be able to apply this same idea to a user’s MySpace or Facebook profile too. You’d have to look into those API’s.