Hi edogpus,
The issue is you need a bit of css to position the background. The current background image in the demo is: http://www.kriesi.at/themes/newscast/wp-content/themes/newscast/images/skin1/bg.png
Which gets repeated. So in replacing it you need to change the css so that it stays centered. At the same time, you'll need to remove the default logo since your background image now shows the logo and blog title.
So in style1.css where you added in the url for the image, change that whole thing to:
body{
background: #fff url(http://www.unikorns.com/wp-content/uploads/2012/09/header-1.jpg);
color:#666;
background-repeat: no-repeat;
background-position: center top;
}
Now, in the same file (style1.css), you need to scroll down to the bottom and add:
/** Remove the Logo and logo link**/
#top .logobg a {
display: none;
}
/** Remove the Social Icons and Search Bar**/
#headextras {
display: none;
}
I added in the search bar removal as well since it covered some of your header image.
For additional customization like this, I would recommend looking into a freelance developer who can take a big list of little css customization's and fixes like this and get it done in a few hours.
Regards,
Devin