How to replace background image in this template?

It's declared via css:

.frontpage #gkHeader {
	background: url('../images/header_bg.jpg') no-repeat center top;
	background-attachment: fixed;
}

so you can change the image by replacing above URL or replace the current image: /templates/gk_steakhouse/images/header_bg.jpg by your own one.

Glad to be of help!