http://www.webpr.gr/joomla/2006/06/ultimate-png-fix-plugin-for-joomla-10x.html

The Ultimate PNG Fix Plugin for Joomla 1.0.x and Mambo 4.5.x gives you the ability to correct "alpha" transparency for both inline and background 24/32-bit PNG images on Internet Explorer (from version 5 up to version 6.x, version 7+ supports PNG transparency natively). It does that by triggering some optional filters in IE using Javascript to achieve alpha transparency.