yeah, it wouldn't in IE6 if you saved it as a png... there are fixes I think - but GIFs should work fine in IE6 anyway, that is, if you aren't too worried about the quality?

Perhaps this code would work better, by the by:
in the body have:
(underneath most of the HTML)
<div id="cornerImage"></div>

then, in the css:
#cornerImage {position:absolute;
right:0;
top:0;
background:transparent url("whatever");}

(this one isn't tested either sorry!)