I don’t have IE6 in front of me but I reckon it’s got something to do with the top 10px margin on the <a> tags. Get rid of that and put a top margin on the <ul> instead. Give the <ul> a height of 11px too.
Well, you’re using "background();" instead of background-image(); for your nav items. You could try changing that out. You also don’t need "transparent" when the background color of the images is the same as the page background (unless you’re putting something behind it?) Also, I think you can take out that scroll function. In any case, if the height of the "li a" were the same as the image height, you shouldn’t have that problem.
Also, as far as I can tell, you don’t need to indicate "list-style-image:none;" if you have "list-style-type:none;" I could be wrong, but I think list-style-none trumps all.