For those like myself that have wanted to customise the screensaver's built in images on the Gen1 ATV... (but not via iTunes etc) The images as you may know are located in:
However, if you try to Cyberduck them (ie SSH / FTP) then you receive an authorisation error.
The answer, though a little drastic for just this purpose, is to remove the hard drive, plug it into your computer, and then access the above directory. Add / remove whatever photos you wish, and voila - the screensaver will now feature your choice of photos instead of those flowery ones.
NOTE - Use JPGs... At least - PNGs don't work.