helengy, another idea has come to my attention courtesy of BDAqua, who is a great helper on these forums. If you are a bit timid about using the above procedure, which involves using Terminal (and I am certain no harm will come by doing it this way), what you can try is to boot in Safe Mode and then try to move these photos off the Desktop.
To start up in Safe Mode, restart or shutdown and at the chime press and hold the Shift key. You only need to keep holding it in until it's clear it has started booting. It will take much longer to boot than a regular startup, so give it time. Also don't attempt to do anything for a few minutes once you've logged in. You'll know it's in Safe Boot because it will bring you to a login screen with Safe Boot in red letters.
After giving it a few minutes with no applications open, see if you can move those photos off the Desktop into some other folder. I'd only try doing a few at a time at first to see how it goes.
This will be very time consuming. The Terminal method should work faster and is completely safe. Just follow the steps; it's really nothing to do.