When it comes to imagery in responsive sites there are a couple of things to worry about; firstly the quality in retina displays (we’ll talk about that another time) and secondly the content within the image itself.
An example of this would be an image of a man stood at a podium delivering a speech in the middle of a large room. Whilst fine at desktop, at mobile size this image will look more like a lollipop stuck in a box.
The answer is responsive content as well as design. As the resolution reduces then the image is changed to be a closer crop, clearly showing the face of the man who is making the speech.
It is important to make sure that image content still makes sense when viewed at smaller resolutions, rather than just being concerned with text.