The only technique I know of that works with absolutely no markup changes is Adaptive Images. It works by routing requests for images through a PHP file which intelligently serves (and creates if need be) images of the appropriate size for the screen width.
Portfolio | Gitmash - 2 views
Icon Fonts are Awesome - 1 views
40 Online Generators for Web Designers Should Bookmark - 0 views
JavaScript best practices - W3C Wiki - 0 views
The Shapes of CSS - 0 views
2More