You can download GIMP here.

A PSP screen is 480x272 pixels. If you make a picture pixel for pixel for the PSP, you get the best possible quality. This is an easy process.

Step 1: Download, and install GIMP.

You should be able to do this without any help.

Step 2: Find a large picture that you want as your new background.

You can use google pictures to find something. The bigger the picture, the better.

Step 3: Open up your pic in GIMP.

Step 4: Scale your image.

Hit 'Alt+I', then 'S' to open up the Scale Image box. Then make sure that both chains are locked, and that the drop down box on right is set to pixels. After that, type in 480 in the width box, or 272 in the height box making sure that both boxes are > or = to 480, and 272.

Step 5: Set image canvas size, or crop your image.

Hit 'Alt+I', then 'V' to open up the Canvas Sizer box. Unlock the chain, and make sure the drop down boxes are set to pixels. Then, make your width 480, and your height 272. After that you're going to want to drag the picture towards the bottom of the box to choose which section you want to crop out. Hit resize, and you're done with step 5.

Step 6: Save your image.

Hit 'Ctrl+Shift+S' to save your picture as a new image. It is important that you do a "save as", because you do not want to save it as a .jpg, because you can loose quality that way. After you hit 'Ctrl+Shift+S', you should see another box that you will use to save your pic. Make sure you find a folder that you can remember, and then type the name of the file you want ending in '.png'. So, the file name should look something like "picture.png". Hit enter twice, and you have your perfectly formatted PSP background.