cdn-srcset-generator

Code:

          
<img src="http://www.fillmurray.com/2000/1236"
     srcset="http://www.fillmurray.com/1024/633 1024w,
             http://www.fillmurray.com/640/395 640w,
             http://www.fillmurray.com/320/198 320w"
     alt="Bill Murray">