Just might be of some help, though please note I am not a coder .
On a local site I have an image (2px by 162px) full width across a Buddypress Default Child theme.
I have a menu in the top 37px as I have disabled the bp admin bar.
the code i came up with is as follows.
`body {
background-color: #eaeaea;
background-image: url(images/headpixf.png );
background-repeat: repeat-x;
background-position: top center;
color: #555;
font-size: 12px;
font-family: Arial, Tahoma, Verdana, sans-serif;
line-height: 170%;
margin: 0 auto;
max-width: auto;
width: 100%;
}
#header {
background: transparent;
-moz-border-radius-bottomleft: 10px;
-webkit-border-bottom-left-radius: 10px;
border-bottom-left-radius: 10px;
-moz-border-radius-bottomright: 10px;
-webkit-border-bottom-right-radius: 10px;
-khtml-border-radius-bottomleft: 10px;
-khtml-border-radius-bottomright: 10px;
border-bottom-right-radius: 10px;
color: #fff;
-moz-box-shadow: 0 0px 5px transparent;
-webkit-box-shadow: 0 0px 5px transparent;
height: 37px;
padding-top: 25px;
position: relative;
z-index: 1000;
width: 1000px;
margin: 0 auto 90px;
}`
Good luck!
