Regarding the image resolution, are you suggesting something like what the blog application WordPress does to images? One uploads an image and WordPress automatically creates different sizes. The different sizes are displayed based on the site visitor's browser settings. That way, mobile users aren't waiting for an enormous image to download over their slow connection.
https://www.smashingmagazine.com/2015/12/responsive-images-in-wordpress-core/
I'm actually not sure what's going on behind the scenes in the examples I'm referring to. I'm going to post them below as a response to @solar so you'll be able to see.