Hi,
The background there is loaded from this CSS file: wp-content/themes/rt_enigma_wp/css/style1.css at line 57.
To change your background you can either change the url there or create a Custom Stylesheet (check my signature) and add the following style (recommended but either one will work):
.backgroundlevel-high #rt-top-surround {
background: url("../images/style1/backgrounds/wood.jpg") repeat scroll 50% 0 rgba(0, 0, 0, 0);
}
Hope this helps