Use this code...
div#rt-header .rt-container, div#rt-header .rt-container .rt-grid-12 {
width: 100%;
}
Then remove the "pull-left" class in the img tag and also the hardcode dimensions.
Then, make the image much bigger than it is (1960px + width)
This is how to create a custom CSS compatible with Gantry 4...
Simply create a file called "<TEMPLATENAME>-custom.css" and put this file in the CSS folder of the template (where <TEMPLATENAME> is the name of the template as seen in template manager e.g. rt_fracture). GANTRY4 will automatically load this CSS file. If you wish, you can also have browser specific files by appending, for example, "-ie9" making the filename "<TEMPLATENAME>-custom-ie9.css"
Regards, Mark.