Future proof image resolution?

1

What is the future proof image resolution for publishing online?
I uploaded all my images with around 800 px width, but viewed on a new 4K laptop screen it looks like this:
screenshot
True, the page is not fully responsive, but still, 800 pixels is almost the minimum width even on a 5 columns layout, on a 4K screen.

Does this mean that to make it future proof, all the images should be uploaded with say, 2000 pixels width?

2

What should it look like? I can hardly see the images. I guess they are in green?
You can always auto size them or make them backgrounds using cover

3

image resolution is recommended to be kept to the minimum, cause this is where performance hits the most.