While I can program (some) in html, I'm not very good with CSS style sheets.
Turns out I needed:
#header-inner {
background-position:center
width: 660px;
height: 233px;
However, before Blogger broke the process, I could add and remove header images with a couple of clicks. Now I have to edit the image with PhotoShop and then edit my template code to match the image size.