Are you talking about the stripe highlighted in my attachment?
If so, go to the Style Manager > Main CSS for the style you wish > search for .tcat
Within the .tcat box, you'll see info such as color codes and one color code with url( after it.
Change the contents of url( to match the path of your image.
So it'll end up being something like...
#FFFFFF url(
http://mydomain.com/images/gradients_tcat.gif)
or just the url part.