Random fullscreen background folder issue

My goal is to have a folder where the resized pictures arrive (through php) so they can be randomly used as a background whenever a new page is opened. Result is an ever changing fullscreen random background when browsing the site.
Everything works perfectly as long as you put/change the photos in a folder through an ftp program.
But this must be done through the admin. The client must be able to upload backgrounds, remove and replace them. WP however makes subdirectories (by date), so all pictures get in seperate folders and can no longer be accessed by the php “random” script, that requires all pictures to be in 1 folder.
Suggestions?
Thanks in advance.