First, you will need to open Picasa. Don’t have Picasa? It is a free program from Google, go here to download it. (Note: Picasa catalogs all of your pictures so they are easily searchable on your computer, it does NOT upload them to the internet without your permission.)
Then on the left hand side of the screen you will find the box below that will allow you to change the pictures to be how you want them. Want more tips on how to make your collage the way you want it? Check out this post. Do you want to add text to your background? Check out this post.
Be sure to change the page format on the left hand-side of the screen. You will want it to be formated for either a standard screen or a widescreen monitor, depending on what your computer screen is like.
You can make any picture in Picasa you desktop background by simply selecting the picture, clicking the create menu at the top of Picasa and selecting ‘Set as Desktop.’