In a responsive design, an image can be displayed with its proper aspect ratio, cropped as the author saw fit. It can be scaled to fill the available space horizontally or vertically, and it can be optionally optimized with a picture tag instead of a img tag to load a lightweight version if the available space is small.

Practical example: a group photo of 15 people smiling at the camera on the official page of some event. You adjust colour, you rotate the picture to get a horizontal horizon, you crop different amounts of excess background on each of the four sides. Then would you like "responsive" tricks to automatically slice away their feet because the screen isn't tall enough? Maybe their heads or a bit of both? Or should they be stretched instead, to disturbing squat proportions?

What kind of pictures look better deformed or mutilated than, at worst, unobtrusively letterboxed?