Since new, my iMac 5K has shown this ghost image problem if an image is left statically displayed for as little as 5 minutes (display off is set to 7 minutes in Energy Saver): a ghost image remains. It eventually fades and goes away, but it’s very annoying when there it is regularly, which it is—every day since I bought it 18 months or so ago. So it’s inherent. The display is set to about 80% of max brightness, so it’s not cranked up overly bright.
I do not know if this is a known or expected defect or behavior, but as shown below, the ghost image of this photograph can be seen in the all-gray neutral gray of the display (there is also one large reflection at right, which is not an issue).
Update: I called Apple and it looks like I have to take it into an Apple Store to be fixed. But the stores are so backed up with appointments that I can’t do that for a week or so as I am going on a trip soon.