Where are your desktop pictures stored in each acct?
I keep mine in a "desktop pics folder" inside the Pictures folder.
I don't use iPhoto saved images for desktops, don't even know if it's possible, as I haven't opened iPhoto in years.
When I accidentally deleted contents of my Pictures folder the other day, including my "desktop pics folder", the Mac started giving me random screen backgrounds each time I booted. Restoring the contents to Pictures folder, and re-selecting the right desktop pics fixed the problem.
Perhaps your Mac is getting confused about exactly where the desktop pics for some of your accts are located, and doing it's best by picking a random image from what it thinks is a reasonable location. You're not keeping background pictures for one acct in a folder belonging to a different acct, are you?