I don't think it's possible to have a different wallpaper on each home screen.
Sorry I don't know of an app for what you're looking for, but may I recommend an app that has wallpapers pre-sized for the Droid. It's called "Zedge" and it's in the Market for free. It's an app that you can download wallpapers and ringtones for free. I have found that the majority of the wallpapers are already sized for the Moto Droid. (split across the 3 homescreens nicely) Some of them aren't quite sized correctly but I have found that they are all high resolution. I don't download that many but the ones I have are very nice. I get a lot of cool ringtones or notification sounds from Zedge too.