The inherent problem of responsive images
As an author, you cannot know about every possible device that may visit your site now or in the future. Only a browser knows the particulars of the device viewing a website; its screen size and device capabilities, for example.
Conversely, only the people making the website know what versions of an image we have at our disposal. We may have three versions of the same image: small, medium, and large, each with increasing dimensions to cater for the anticipated screen size and screen density eventualities. The browser does not know this. We have to tell it.
To summarize the conundrum, we, as the website authors, have only half of the solution, in that we know what images we have. The browser has the other half of the solution, in that it knows what device is visiting the site and what the most appropriate image dimensions and resolution would be.
How can we tell the browser what images we have at our disposal so that it may choose...