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:

Code:
#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.

Code:
#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.