On a standard default vb theme, the category strips use this:
#869BBF url(images/gradients/gradient_tcat.gif) repeat-x top left
The first is the color of course, the rest is the path to the image, and the stuff at the end tells your browser what to do with that image.
http://www.w3schools.com/css/pr_background-image.asp
So, if you wanted to test an image, name it the same as the original and upload to that directory, allow overwrite. Refresh forums page.
Easy huh?
Good luck...