Header Background Image not Stretching

OK, here's (another) weird one: I have a header module background image which, when the site is offline (and restricted to admins) and I'm logged off, shows perfectly in the login screen. I can resize the browser window any which way I like, it always adapts perfectly. However, when I log in the same header image doesn't stretch. When I resize the browser windows I get a while edge on either side of the image.

the weird thing is that, originally, it worked fine, but then I had to switch on the debug mode for the site and when i switched that off again, the image issue came along...

Any ideas? Thanks!

Hi

Please tell me the url to the page I'll be able to see this problem.