I actually did a work-around. Using the original header_bg.jpg to get the dimensions of the image, I created my own, slightly less wide, with white space on the left-hand side. I uploaded it. I then tweaked the logo directive in stylesheet.css by adding "margin-left:100px;". When i brought up my store, logo and header image were side-by side, with nice margins on either side of the entire header. In other words, with a little thought and effort, I got the desired result, though I'm not sure how elegant a solution it is.

Diggy