Thanks, I'll look into that.
In the mean time I've just made a backup of the File_Attribute table in the pluto_media database, then removed all entries that assigned the ScreenSaver attribute. After that I selected the pictures I wanted for the other set in the webadmin.
So to switch sets I have to remove the entries from the table and restore the backup associated with the set I want. (The restore only adds non-existing entries, it will not overwrite any removed entries).
It's a quick and dirty hack, but for now it enables me to do what I want (party is on friday
. As soon as I have time I'll see if I can make a more elegant solution.