you can add your own wallpapers to the stock wallpapers gallery easily by creating a "wallpapers" directory in /system/media/ and changing the folder's permissions to drwxr-xr-x. just look at the permissions of the other folders in that directory to see the configuration pattern.
now you just need to make a thumbnail image of your wallpaper and copy both to the /system/media/wallpapers directory you just created and change their permissions to rw-r-r.
open your wallpaper gallery and your wallpaper will show up at the end of the preview strip.
note: the wallpapers will also show up in your gallery app, but they can be deleted safely from the gallery without deleting them from the system.
