Doubletake dynamically updates the src of your images based on the browser width. Start with a small, mobile-friendly image in your HTML. Doubletake will use a defined set of breakpoints to update image SRCs when necessary.
I’m using a couple of tools to help me resize the images on this page – a local copy of SLIR and the src.sencha.io web service. If you aren’t comfortable resizing images on the fly, you could simply host multiple copies and then set the Doubletake breakpoints to the sizes you have created.