I would add you may use more than up to 10 standby screen pictures.
I have more than 50 of them. Thanks to Booxstor for his very nice collection.
If you want more than 10, just continue numbering screen_saver10.png, screen_saver11.png, ... I guess, it should be OK till 99, maybe 100 and more will also work...
There is also script to dynamically change startup screen, but I haven't managed to download it and test it.