got problems with changing the body background

Hey everyone – i’m working on my first wordpress page at the moment. Everything went fine till now but i got some Problems at changing the Background of the Body. I want to set a picture as a background but it seems that something overwrites codes like:

body {
background-image: url(‘http://www.link-to-your-image.com/img/image-name.jpg’);
}

I dont know how to explain that correctly, if i put this code in the style.css the background stays the same but i can see the new background image for a second behind the header image..?

The URL is: http://www.baracca.ch/wordpress/die-bar/
and i’m using the Growler Theme from Progression Studios.
The Theme offers a function where i can edit the background colour right in the theme but i cant change it to an image.

Anyone knows how to solve this? Thanks for your help:)

Step to step instructions would be awesome as i’m pretty new to wp.