It's a very cool idea, we've used a similar technique to create the left hand navigation of our website http://pollenation.net. This uses pil and javascript and adds events for the rollovers too. We did try the sifr flash image replacement but flash didn't play particularly niceley. Be wary that when using onload, all of your images and stylesheets and other assets will load before it's triggered.
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It's a very cool idea, we've used a similar technique to create the left hand navigation of our website http://pollenation.net. This uses pil and javascript and adds events for the rollovers too. We did try the sifr flash image replacement but flash didn't play particularly niceley. Be wary that when using onload, all of your images and stylesheets and other assets will load before it's triggered.