They're actually located in /system/app/Launcher.apk
If you can get your modification to work, I'd love to see it. I tried to add in a couple of wallpapers I found with a MetaMorph theme of my own, but they don't show up in the wallpaper gallery.
The MM theme I have has 2 separate files for the wallpapers, too. One, I assume, is the smaller preview on the bottom. Just wondering so I can see if I really can inject new wallpapers in there.