I'm looking for a Mac user with the new 5K Retina Display Monitor to do a test Web Page test on how a graphic appears ? The image I'm using is basically 4K max in size I may have upscale the image unless someone else knows some other tricks because if an Image is suppose to be 2x or 3x and the maximum the image is, is 4K you have to fill in the gaps. I'm afraid too much upscaling and everything will turn sour on Retina Display Not sure how 4K/5K images look on a PC, what I do know is on a PC a 2x or 3x image isn't required, as far as I'm aware !
The web is the web. Why anyone would even bother to use a 4k image is beyond me. Load times on that page are going to be awful. The rest is out of your control, anyway. You just don't know which browser will be used and how its scaling options are set nor do you know how the images may be mangled via the CSS formatting or other means like server-side processing. I seriously think you are obsessing over a non-issue.
How would you tackle the situation, by not using 4K image and instead find another means if the user visits the page on a 4K resolution monitor ?