All of the backgrounds I have encountered have only the left side customized. I calculated the width between the content and the edge of the screen and came up with a value of about 115 pixels. Keeping in style with my avatar I found a cool looking pepper in a free stock photography site and decided to modify it to size.I also wanted to add something to the left hand side. I had to take several guesses how far to place the graphic and it seemed to work (it was about at 885 pixels from the right). It probably totally disappears from view if one has a widescreen, but that's OK as the right graphic remains.
Here is a list, in no particular order, of sites you can visit to obtain or create your own background.
webdistortion: uses some neat backgrounds and suggest the best colours for you to use with each of them.
Boinblog: download a photoshop template, customize it.
TwitterBacks: free templates or get your own and resubmit it to the site.
savesavesave: a bunch of free twitter backgrounds.
croncast: probably the site most referenced for creating a background.
Twitter Patterns: very pretty backgrounds, a must to see.
Twitter Templates: a bit disappointing, but that's just my opinion.
Twitter Gallery: they have a very nice gallery and it allows you to upload the background directly to your twitter account, or manually if you prefer.
The Closet Entrepreneur: instructions for creating your background using powerpoint.
TweetStyle: nice free backgrounds and they will also custom make them for you.
limeshotmarketing: very nicely made free backgrounds using the twitter bird. There is even one with a transparent background.
Twitter Treats: artistic backgrounds, check it out.