Hi, when you use the wallpaper app just save the ones you like to the camera roll. Then using your FTP client connect to the iPod Touch. Navigate to the root/media/dcim/100apple and you'll find them all there.
Just download them into you PC/Macbook and put them to the photo's folder that you want to synch your ipod Touch and iTunes with.
Once they synch you can then use them as wallpaper on your iPod
Cheers!