You can alter the CSS to suit the needs pretty easily. In your theme files is a folder called css. Inside that is style1.css. Look for the following:
background: #fff url(../images/skin1/bg.png) repeat-x 0 0;
That image is this: http://www.gezgindergi.com/wp-content/themes/newscast/images/skin1/bg.png
The most basic change you could do is to download that image and use it as a template for the image header you would like to use. Then replace the css in the style1.css above with:
background: #fff url(http://......yourimage.jpg) repeat-x 0 0;
Just make sure to replace the http://... line with where your image is located on your server.