I love working with PNG’s because they offer true alpha transparency, with the downsides being that IE6 doesnt support alpha transparency and their file size can be not so great compared to a .jpg image. Sometimes you will see a difference of up to 100k between a .jpg and a .png. I have seen websites that use optimized .png images and have always wondered how they keep them so small.
First I tried using Yahoo Smush.it – but I couldn’t shave that much off the file size. After a bit of Googling, I came across this great little tool for Mac called ImageOptim. Its a free download, with a simple drag and drop gui for optimizing not just .png’s, but also .jpg’s and .gif’s. It will shave off any unused KB and not affect the quality.