If you log into your Twitter account, go to the Settings link at the top right hand corner of the screen, you should find an option to change the background image of your twitter page.
How Do You Put A Background On Twitter?
So when your logged in, there is a task bar at the top. Click 'settings' then there are tabs at the top of the page. Click design. Click a design you want, or click change background image to put one of your pics on the background.