Site review request

I've spent little time modifying this Dreamweaver template to hack out a
demo for a proposed site design. The particular issue I'm asking about is
that the masthead, consisting mainly of three images in as many layers, has
a white background an looks continuous, as I wished, in Internet Explorer 6,
but in Gecko/Mozilla/Netscape 7, the gray background image shows through in
the middle third. Can anyone point out what I'm missing? Thanks.

http://www.wssu.edu/forstudents/sanfordw/wssutest1.htm