I had a similar situation.
If you look at the non-WordPress part of the site
http://everydaypays.com
I wanted to match the header of the blog.
I replaced the top part of the gradient background with cloned material and cut out the part with the image to use as a header image on
http://everydaypays.com/blog/
Then I added this code that Juggledad posted earlier combined with setting a negative top-margin for the menu division
Code:
div #menu1 {background:url('/blog/wp-content/themes/atahualpa/images/atahMenuback.JPG');
margin-top:-54px !important; }
.rMenu { background-color: transparent !important;}
(Obviously I'm not done. Just getting a similar starting point to the rest of the site before I meet with the site owner)