@n1ckspace did you know there’s a new version of this theme? Check out the demo – http://demo.appfinite.net/optimal/
If you prefer the newer version then I’d start with it instead.
For the older version that you’re using, the image you see can be found on line 2245 in your CSS –
.slider-wrap {
background-color: #f5f5f5;
background-image: url(images/slider-bg.png);
background-repeat: repeat-x;
}
The only way to extend it is to either recreate or stretch the image in an Image Editing Program like Photoshop. But if you like the newer version of the theme, then definitely make the upgrade.