Ok what a pain in the ......., updated to windows 10 v1903 and the lock screen issue has re-appeared. I get the issue that if I choose slideshow for the lock screen "you need more than one picture in this folder for slideshow" This has happened to me a number of times over the years, usually after a version update. I find the best solution to be:
Copy all of the pictures you want from the folder you have in (e.g. C:\user\Pictures) and paste them into C:\Windows\Web\Screen (this is where windows stores the pictures you normally select for the lockscreen slideshow and spotlight.
You will find 5 Jpeg files in this folder which are the default images, if you don't want these then you need to delete them.......Oh dear! the permissions for these are set for trusted installer, so you can't just delete them OMG @!?**!!. So you need to know how to change the permissions........Oh my god this is painful, I know!!
You have to do one picture at a time,
- right click properties,
- click security tab,
- at the bottom click Advanced,
- at the top owner = trusted installer, click change,
- new window, click advanced
- new window, click find now
- scroll down, find either admistrators + your user name, or user + your user name
- click on your choice and click ok
- back to previous window, your choice is shown, click ok
- back to previous window, owner has changed, you must click APPLY
- new window click ok, then ok again.......OMG!!!!!!!
- now you are back to properties window......you are not finished yet!!!
- you need to click advanced again
- now click Change permission (bottom left of window)
- find your user name on the list that you chose for the owner, and select it
- you now have the option to Edit on the buttons below, click it
- new window, tick full control, then ok
- back to previous window, you must click APPLY
- another window to confirm, click YES !!!!!!!!
- then click ok, window closes, click ok again to close the properties window
CONGRATULATIONS!!!!!! you now own the file and you can delete the mother @*!!er
When you finish doing this for all five of these files, and you haven't launched your PC out of the window...... you need to select this folder in the lockscreen personalise options.
So just to remind you where you did all of this fine work C:\Windows\Web\Screen is the folder you want.
Your windows 10 lockscreen slideshow should now be fully functional.