CSS background image missing in CHM

Hello,I'm using the 1.7.20 htmlhelp stylesheets to build a .CHM (HTML Help) file. Recently I added a background image via the separate CSS file:
body {	...
	background: #fff url(images/bg-side.png) repeat-y;	...
}Even though I have the htmlhelp.enumerate.images option turned on though, the background image is not included in the either the generated hhp file nor the chm file. I suspect the css file is not searched for references to graphics files. Has anyone found a way to work around this? Perhaps some method to give an additional list of graphics files that should be included?

Thanks,
Ken