http://apiwiki.twitter.com/REST%20API%20Documentation#AccountMethods
update_profile_background_image
Updates the authenticating user's profile background image.  Expects raw multipart data, not a URL to an image.
URL: http://twitter.com/account/update_profile_background_image.format
Formats: xml, json
Method(s): POST
Parameters:
• image.  Required.  Must be a valid GIF, JPG, or PNG image of less than 800 kilobytes in size.  Images with width larger than 2048 pixels will be scaled down.