In my experience the background image spreads over all the widgets in the sidebar as if one image is behind several windows. It looks really nice. But if I understand what you are describing, I’d put the image I wanted in a widget at the top of the sidebar and then style that widget to have a transparent background and no margins or padding. For example:
#right-sidebar #text-5 {
background: transparent !important;
margin: 0,0,20px,0 !important;
padding: 0 !important;
}
If you don’t really want it in the sidebar widgets but in the container, I’m sure Tom will have great advice.