desktop image disappears, restarting fixes only temporarily
Under 'System Preferences' I choose a background/desktop image I've used many times. Shows up fine, no problem. Later the same day - sometimes almost immediately - the image disappears and there's just a blank desktop. If I restart the image shows up again, but will shortly disappear.
This is a small matter, but I find it unsettling to be staring at a big blank screen.
Any help much appreciated.
MacBook Pro 15″, macOS 11.4