Thank you so much for your tips, Thimothe!
I want the logo to appear over the background header image, no matter how much you enlarge the browser window. It needs to sit within that 1200px width, as if it were part of the header image. I can't put the logo right on the background image because I am using this WP install with Quark child theme to run 3 sites, all with the same header background image and layout but a different logo and content. I do want the site to be responsive, so the logo doesn't need to be absolutely placed. If you have suggestions for achieving this effect in another, feel free to suggest!
I removed the left margin on #masthead in my child theme css, but it didn't bring the logo over onto the header image. I didn't find #masthead in the main Quark css stylesheet.
I have been editing my css in Dreamweaver and uploading manually (my normal workflow outside of WP), and it's a pain. I can build this site outside of the WP framework easily, but the specs for this project call for WP, and I'm really struggling with the WP backend files and pieces/parts. I'll try Firebug and Chrome tools, thanks for the tip!