Adding background image with css

Are you familiar with CSS? Just add a background-image to your body, like so:

body {
background-image: url('image.jpg');
}

More information about backgrounds here. Note that your slider and header-wrap have backgrounds that will cover up that body background image.

Exactly which CSS class to choose depends on where exactly you want the background image.
If you want it in the header (only), then put it in #header , for example:
header {
background-image: url('image.jpg');
}

PS. I have not yet looked into this particular CSS file, so the code is just an example.
Find the header id in the CSS and add the background-image line to its existing settings.
Upload your "image.jpg" to the theme folder and replace "image.jpg" with the actual file name.