FYI, png is not that well-supported. I did some cross-browser work a few months ago and there were issues with IE 5.5 or 6.0 (can’t remember now). The images would not show on the browser. Note, there is a way around this problem but it involves some code hacking. I suggest going with gif if you need the transparency effect. The gif and jpg formats have been around longer and are better supported in browsers.
With regards to performance, you may want to bring this issue up with your hosting provider. I pulled up your site and checked the download times of the individual page elements. The timings are significantly slower (2 to 3 times) than the hosting provider (http://www.SimpleHelix.com) I am using to evaluate Magento. Both servers are located in California so there is not a distance factor here.