yes, in the stylesheet
find this:
#header {
background: #90a090;
border-bottom: 3px double #aba;
border-left: 1px solid #9a9;
border-right: 1px solid #565;
border-top: 1px solid #9a9;
font: italic normal 230% 'Times New Roman', Times, serif;
letter-spacing: 0.2em;
margin: 0;
padding: 15px 10px 15px 60px;
}
change the background line to this:
background: #90a090 url(my_header.jpg) no-repeat;
make sure you change my_header.jpg to the name of your header file.
also add a line
height: 150px;
and make sure instead of 150 its the height of your header image.
put the header image in the folder with the theme's php files - that will work with the background code above
You may have to play with the padding.
padding: 15px 10px 15px 60px;
the order is top right bottom left
If you need to adjust the text that appears in the header area its in header.php