Transparent PNGs are great, but they come at a price: file size. Wouldn't it be nice if there were transparent JPEGs instead? Unfortunately there is no such thing, but ZorroSVG gives you the power of transparent PNGs for the size of a JPEG. It achieves this by converting your PNG to an SVG which uses the compositing capabilites of SVG to create a masked bitmap on-the-fly using JPEGs for both the image and its mask - achieving most of the time a much better compression than PNG. Since all the compositing and masking happens inside the SVG there is no JavaScript required. And it will work on any current browser.

http://quasimondo.com/ZorroSVG/