High Sierra giving me tired eyes

That's how I have always thought of it.

When you contact Apple, just be prepared for a lot mundane and ultimately fruitless instructions at first. Fully 50% of their Support calls are resolved by answers like "you need to plug it in and turn it on" so you have to jump that hurdle first. Eventually you will get the attention of someone who will collect some data from your Mac and forward it to Engineering, after which your concern will appear to disappear into a black hole for a long time. The reason I know is that I had a very similar complaint some months or years ago: nonspecific visual fatigue after using the Mac for a while. I couldn't even explain it as well as you did. It was eventually addressed in an update, after which Support contacted me to ask if the change improved things. It did. No more tired eyes.

So... be patient.

Image rendering is so subjective it's impossible to say what might be wrong.

Finder should not quit though. Please determine if it also quits unexpectedly in "Safe Mode": Use safe mode to isolate issues with your Mac - Apple Support. Using a Mac in that mode is so visually objectionable that you are unlikely to find it comfortable to use for any period of time, but determining if Finder quits or not may help advance troubleshooting.

This is something you should definitely contact Apple about: Contact Support. There are many possible remedies including calibrating the iMac's built-in monitor. Read Color Preferences - Apple Support for one example. Others may be found in Accessibility Display preferences - Apple Support.

Since the same happens in other user accounts the issue is systemwide. You might try reinstalling Mac OS from Recovery and see if that helps. Also try John Galt's advice re color preferences and Accessibility.

Create a fresh user account and drag some images into it to test. If it doesn't happen in the fresh user then we know we need to troubleshoot your user.

That interaction is a very plausible explanation, and something that Apple will definitely be interested in. Please contact them and explain what's going on.