Fundamentals of Responsive Images

People don’t want to wait around longer than necessary, though. Any benefit you get from a great image vanishes once someone’s neck begins to tense up as the loading bar slowly creeps from one side of the URL bar to the other.

Images also lose their emotional impact if they’re blurry and someone has to squint to see the subject.

If you take an image that looks nice and crisp on a phone, then share that same file on a big desktop screen, it’s going to look fuzzy. Switch it around with a nice, big image that looks great on desktop, and somebody looking at the same file on a phone will grow impatient, waiting for the file to load.

We want the best of both worlds: images that look great no matter which screen they’re viewed on, while loading as quickly as possible.

Source: Fundamentals of Responsive Images