It might be obvious to most, but if you care about photo quality, and want them to look their best, then resize & crop them to 480 x 272, then unsharp mask before putting them on your PSP. The difference is very noticable, because the PSP will blur any photos bigger than 480 x 272.

If using Photoshop, sharpen the resized images slightly. Filter > Sharpen > Unsharp mask > Amount 60%, Radius 0.3, Threshold 0

Some photos you might need more or less Amount. Never less than 40% because you won't notice it.

The end result of all the above is super-crisp, brilliant impressive images that fill the PSP screen at 100% zoom, with no blurring. And they are small filesize too.