You can't do it with a template override since there is no abilioty ot change header background image in this template.
So, you have to do it with CSS...
body.menu-home #rt-header {
background-image: url("yourimageurlgoeshere");
}
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"
We have now made it even easier to join RT by lowering prices (you can now join just for a single template).
Out priority will always be to assist paying members.
Regards, Mark.