Header and embedded styles

I finally got somewhere in my quest to change my header image. I successfully replace the default kubrickheader.jpg file with one of my own with the same name and dimensions. It shows up, but only behind the default colored box. I looked at the source code and found something that turned out to be in the “Embedded Styles” when I checked it with the Web Dev extension.
This is the line I mean:

<!–#header { background: url(‘https://***.com/wp-content/themes/default/images/header-img.php?upper=FF0000&lower=FF0000&#8217;) no-repeat bottom center;

When I change the “header-img.php” in the line to “kubrickheader.jpg”, the image shows up, but of course when I leave the page the changes revert. I saved the edited embeddedstyles.css file and uploaded it – nothing. I’ve read through here and Google for hours now and can’t find anything.

Where can I find the embedded styles in WordPress? I’m about to go nuts and desperately need some help.

Thanks in advance!