From DJ Loose:

Just another hint.. as you probably already know, the Pre has the same screen resolution as the iPhone (boo..) but anyway.. Option "A" would be a quick Google search for "iphone wallpapers", which will get you plenty of sites with plenty of results already sized and ready to go for your Pre. Option "B" would be to use Google's newly-renovated "Advanced Image Search" to specify the "exact image resolution" to be 320x480 of course and then you can use the "match any of the following" for something like "abstract, 3d, nature, etc".. whatever you want.

So here is the link for search on Google: Wallpaper 320 X 480