Intenet Explorer 6.0 and older versions do not support transparency for png images. We've implemented a workaround to make these images look good in older versions too. However this option is disabled by default
Go to Menu->Tools->Options->HTML->Include pngfix.htc which allows IE to properly render PNG alpha transparency to allow IE to properly render PNG alpha transparency.
Note that many of the Web Builder objects (like shapes, drawing tools, images, navigationbars) generate transparent PNG files so if you want your website to be compliant with IE 5.5 and 6 you should enable this option.