Yes.
I take it you have jailbroken your touch?
If so when do you know how to access the root folder using an SFTP program? Either create a new summerboard theme by copying out a theme folder from the root folder or just drop a new wallpaper into the theme that you use.
The wallpaper must be 320x480 px and be .png format, in OS X you can simply rename a .jpg to .png in finder and it will work. It also must be named Wallpaper.png (casing is needed as well)
The 'theme' wallpaper when using summerboard is the wallpaper that is used when the ipod is unlocked.
To have a different wallpaper when it is locked you only have to sync an iphoto album with your other ipod touch sized wallpapers (320px by 480px), then in 'settings' go to general>wallpaper an select one from this album. This wallpaper will be for the locked screen.
Any questions?