Optimal photo dimensions?

We are using Google Drive to store and link to our photos and whilst they look great in mobile mode, they don’t look so sharp in tablet, and especially in desktop mode.

So, what is the recommended dimensions for photo uploads in order for them to look as good on desktop as they do on mobile?

Posted this same question here. It’d be great to have a guide so that we can create multiple images for different devices or scenarios.

This might help.

@Jeff_Hager the article doesn’t really tell anything about the resolution of the image. How many pixels to choose.