Hi Terry,
3 things have to conspire for this to happen.
a) IE6 (it doesn't happen in IE7 or
b) the graphic must be a PNG
c) the graphic must be "hidden" when the page is first opened.
Changing any of these factors cures the issue.
I know there are some people out there still using IE6, so it's worth testing in that - but there's a very strong move on the internet now (including microsoft) to do everything possible to encourage users to upgrade to IE7 or 8, or better yet to a browser like Firefox.
Apart from the display issues (and IE6 has _lots_ of those) there are also numerous security issues. Frankly anyone still using IE6 is asking for trouble on the security front.
Changing the graphic to a JPG is an option for you - although you lose the transparency support PNG's have so it's a small step backwards.
Cheers
Bruce