If you want to change your Twitter background regularly and automatically, You can use Flickr My Background a little tool. It will put you latest Flickr image in your twitter background.

Flickr My Background check your Flickr for new photos and sync every hour with Twitter. You have to give your twitter username and password to Flickr My background. Then You have to validate Flickr account to complete the process. Now if you upload images or photos in to Flickr, the latest image will be appeared as your twitter background.