Internet + Potatoes = Pure Awesome
This extension most images to potatoes. You can adjust it to alter only some images, some background images, or alter all background images. As of the current version, it does not yet change iframes, flash elements, HTML5 canvases, and most ads.
Image from HowBigIsMyPotato.com. Used with permission.
The source and changelog for the program can be found at https://github.com/FracturedRetina/PotatoIt.
If you have any suggestions for features leave a review or open an issue on GitHub.