Just having thousands of files on the desktop will cripple the machine. They don't even need to be open. You have to go into another user area where it is not trying to use that Desktop folder for the desktop but where it is just a normal folder, then get the majority of those files out of the Desktop folder. Either that or come up with some other way (such as single user mode) to move those files so they are not part of the active desktop in the account you want to use. I was presenting two options but there are likely others. The sudo command would likely work and you could combine it with a wildcard and .jpg so it just moved photo files.
I just went to a non-admin account on my computer. I clicked on a folder in my admin. account which was showing the prohibitory sign for me in the other account. Got info. Showed details. Clicked on the unlock icon. Clicked on the "group" which was set to my main user. Dragged down menu to non-admin user account I was using. It asked me if I wanted to change access and presented me with a sysadmin user and password prompt. I didn't want to do any more but it looks like you can do it from another userID which was what I was remembering.
OK, that should probably do it. But still leaves unanswered just what would be moved, if they're all over this crippled Desktop -- and in what form we still don't know, opened or unopened.
If they could all be moved into a folder e.g., one titled "Pictures," you could log into another user and move them into Documents in the original user. For example (the user "helengy" is hypothetical):
sudo mv /Users/helengy/Desktop/Pictures /Users/helengy/Documents/Pictures
But, and this is a big but, since the Finder is crippled for the Desktop, I don't see how you get them into another folder, Pictures in this case, in order to move them. And if you could do that you wouldn't need to log in to another user in the first place.
And you could do a sudo mv from another user, but just what are you moving, the entire Desktop folder? If you copy/move the entire Desktop folder somewhere, that still leaves you with a completely unmanageable folder in that new location.
I think the biggest obstacle is finding some way to isolate them, get them caged up, if you will, in preparation for a move, whatever kind of move that might be. Here I'm stumped.