You mean the right size? Just get the program you use to put it in 480x272. That's the psp's res.
If you want to crop an existing image into widescreen, just use Microsoft picture manager or paint to crop it to 480x272. If you want to squish it, that's a bit trickier. The easiest is probably with paint.
Well you can either take the memory card out of the psp, insert it into a card reader in your computer and copy the picture into the folder in the memory card named PICTURE (I think).
If you don't have a memory card reader you can connect the PSP to your computer via a USB cable and copy the picture in the same way.