You could use the alpha image loadeR to display the image but it won't work on sptites.
There is also the pngfix js you could add to get them working.
Or you could just use transparent gifs for IE6 only and match the matt background as close as possible to avoid pixellation. Ie6 is low usage these days so i would tend to swap transparent gifs (or 8bit ined transparent pngs). They don't look as good but hey its only ie6
What have you tried so far?