Check it out all Edit your own backrounds!

http://thepspimp.com/edit

of course you need PSPersonalize to get the pics on your psp

and in case you like to edit pictures offline

http://www.irfanview.com/

free program, make sure you keep your resolution at least or under 300x170 other wise your PSP won't load the backrounds

First one is pretty nifty, but the resize option distorts the image o.O