Fluid Images

 "... set a max-width of 100% on all images on my website:

img {
  max-width: 100%;
}

This solved the problem beautifully. Instead of just rendering at its native width and overflowing its containing box, the image would render at its native dimensions as long as its width didn’t exceed the width of its container. "

http://unstoppablerobotninja.com/entry/fluid-images/