I created the collage above using Picassa - free to download from Google. It is a great program for organizing your photos, doing some fixes and uploading them for web use and much more.
Picassa automatically finds all the photos on your hard drive and then you can click (while holding down the control button) to add photos to the area in the lower left bottom of your screen (press the little thumb tack button after each picture- to keep it there). When you have all the pictures you want in your collage - click the collage button (bottom of screen). Play with the different types of collages till you get one you really like- so many variations possible! You can even add text - I added my web site name. Then save them and upload to twitter (design tab- change background) or anyplace else that you want to customize. If you need more info or something here is not clear- leave a comment.You can see it at: divine3designs on twitter
If you used this to make your background- leave a comment- I'd like to go see it!