It took me a while to figure this out as well as we wanted a background image for the whole page, so we put the image code in the #MainWrapper like below:
#mainWrapper {
background-color: #FFFFFF;
background-image: url(../images/back.jpg);
background-repeat: repeat;
text-align: left;
vertical-align: top;
border: 1px solid #000000;
}
But we ran into your problem as well. What you have to do is comment out the white background color on the two sidebars.
#navColumnOne {
/*background-color: #FFFFFF;*/
padding: 0.25em;
}
#navColumnTwo {
/*background-color: #FFFFFF;*/
This worked for us. You would just need to put the image code in the area stated above for the images to only show up in the sideboxes.