Kumaro, go to your admin panel then go to style>modify>fonts/colors/etc> then under Category background color just use this:
#FFFFFF" background="{imagesfolder}/yourimage.gif
change yourimage.gif to whatever the image is that you made. Make it 25pixels high by 1pixel wide. you can change the #FFFFFF to your background color for your category background that way it atleast has a good color before the image loads. Also if people browse without images on then they will atleast get a good background color