Serving the appropriate image to users based upon the particulars of their device and environment has always been a tricky problem. This problem was highlighted with the advent of responsive web design, the very nature of which is to serve a single code base to each and every device.
As an author, you cannot know or plan for every possible device that may visit your site now or in the future. Only a browser knows the particulars of the device using it (screen size and device capabilities for example) at the moment it serves up and renders the content.
Conversely only the author (you and I) know what versions of an image we have at our disposal. For example, we may have three versions of the same image. A small, medium, and large: each with increasing dimensions to cover off a host of screen size and density eventualities. The browser does not know this. We have to tell it.
To summarize the conundrum, we have halve of the solution in...