Adaptive Image

The Adaptive image module provides device-appropriate versions of images from your fields.
You can activate adaptive images by adding an adaptive effect to any of your image styles.

The used technique is derived from http://adaptive-images.com/ by Matt Wilcox.
The hard work is done by Drupal core no need for any additional rewrites.
If image styles work on your website, this module will too.

Hint: Javascript is needed on the clientside to determine the target resolution

For additional information on Responsive Webdesign and Fluid Images check out
http://www.abookapart.com/products/responsive-web-design/
http://unstoppablerobotninja.com/entry/fluid-images/
http://24ways.org/2011/adaptive-images-for-responsive-designs/