Hi,
A) In your theme folder under CSS > light-skin.css you'll find:
#header {
background-image: url("../images/skin1/gradiant-dark.png");
background-position: left bottom;
background-repeat: repeat-x;
}
or something along those lines as long as it is #header and something between brackets.
Now change the url("LINK HERE") to something different. Like this:
#header {
background-image: url("YOURLINKHERE");
background-position: left bottom;
background-repeat: repeat-x;
}
If you'd like a repeated background just replace that.
If you don't want a repeated background use:
#header {
background-image: url("YOURLINKHERE");
background-position: left bottom;
background-repeat: no-repeat;
}