What are the dimensions of the image you are trying to use for the header background?
I ask because – using Firebug – when I changed the “width” parameter of #ja-header (line 314 of template.css) to 100%, as follows . . .
#ja-header {
background: url("../images/mainnav-bg.png") repeat fixed 0 0 transparent;
height: 175px;
width: 100%;
}
. . . the image stretched the entire length of the page, but there was a “stitch” effect where the image started again (I forget if there is an “official” term for that). In any case, I’m wondering if, perhaps, the image you want to use for your header background isn’t wide (or “long”) enough.