Solved Image retention / persistence iMac 5k

Seen a few threads on this.

I had the same issue after my screen saver didn’t kick in and the computer was on for 10 hours... Faint image of previous windows especially highlighted against grey backgrounds.

Honestly, if you have the patience - the Apple fix for this does work.

Simpley set your screen saver (the spidery lazer things) to its most beefed up setting And let it run for LONGER THAN your screen was open on the images that retained! For me - i left it a good 18 hours to be sure. It complety fixed it!

have faith and be patient.