Responsive Theme/Appearance/Header Image/Fit

New poster here. Please forgive me if this post should be in another area.

I have installed the Responsive theme and uploaded a custom header image (you may recognize the image from the Weaver II theme):

http://www.thomastunespianos.com/

If you look at my page, you will see that my problem is that I can’t fit the image to match the length of the colored stripe underneath it (right-hand side, not quite wide enough). I was on a page that gave the width of the image in pixels a moment ago but can’t find it now to adjust it. Is there another/better way of doing this?

Thanks,
T