Practical Responsive Images
Responsive Websites need Responsive Images
Crop to Point of Interest (Art Direction)

As the dimensions of a particular image are shrunk down, the main focus of the image may be de-emphasised, and as such we could consider cropping the image to improve it's framing.

In this example, I’m applying different at-request-time crops depending at different min-width’s, with html scaling between those breakpoints.

min-width: 850px : …?pcrop=700,0,2500,800
min-width: 600px : …?pcrop=1300,0,1900,800

and for images requested smaller than that, I swtich out to use a different, focused image request.

Demo: Crop Apples