What does Gimp have to do with Twitter? If you haven't already, first of all see Open source madness - Customizing my Twitter page with Gimp.
Now, isn't this overkill, you may ask, using Git to version graphic design work?
Well, not really. The whole idea of using Gimp is to have a kind of source file, which then generates your Twitter background. Then, say you want to go back to a previous version, if you are using any kind of version control system, you can go back in the time machine and generate from there. We'll use Git.
Open up a terminal on your working directory. Perform the following commands: