I assume you know how to access the files on the Touch?
If so, create a new folder on your desktop, MyTheme or something, put the wallpaper in there and save it as Wallpaper.png
Anyway, once you have that, navigate to /var/root/library/SummerBoard/Themes and drop you folder in there.
It should now appear in the SummerBaord theme list.
To get it as the lock screen wallpaper;
Navigate to /Library/Wallpaper and drop it in there, following the naming convention, copy and paste it and rename one to include the .thumbnail extension.
You can now select your wallpaper from the main settings.