Screen Impression

Hi ncvipi,
I understand that you are seeing some image persistence on your iMac. I can go over why that is.
The reason is that newer displays have In-plane switching (IPS) built into them. This is generally used so you can see the image from different angles and not just straight in front of you. One thing that can happen with this technology is that when images are left in place for long periods of time, they will look like the image you posted. That is referred to ghosting or image persistence. You can fix this simply running a screen saver for a short period of time and that will go away.
Take a look at the article below for more details.
Avoiding image persistence on Apple displays
https://support.apple.com/en-us/HT202580
Have a good day