You can set a background image for any element you want.
You just ahve to assign the element an ID or class and set the background image.
Open the head.tpl and you could use somehting like this.
<table cellpadding="0" cellspacing="0" width="100%" align="center"
id="headBG">
then in your skin1.css you could use
#headBG{
BACKGROUND: url(images/
whateverYourImageIsCalled.gif); background-repeat: no-repeat;
}
set the properties to whatever you want for repeat and placement.
make sure you upload the image to the skin1/images directory.
here is an excellent site for learning CSS
http://www.w3schools.com/