Well...the IE6 it is not a very good friend with *png format. You might want to save (and change in the StyleVars) your logo. I suggest to save it as gif but bare in mind you might have some problems with the margins of the visible elements. You can combine it with the header and get some good results or you can change it to a gif wih transparency but you will need to experiment that for yourself.
P.S. - kind of dizzzy atm because of some beers i had. I hope i make some sense