http://blog.brenelz.com/2009/01/27/png-8-alpha-transparency-screencast/
PNG -24 is what most web developer ’s use for transparency on the web. It tends to have large file sizes but provides excellent transparency. Using PNG -8 or GIF for transparency doesn’t work near as well… at least normally! There is actually a way to use PNG -8 transparency that is better than PNG -24!



