How to fix background image?

I uploaded my background image, but it moves as I scroll down the page. I understood that I need to append this code, "no-repeat top left fixed", but I could not find my image url in the Stylesheet.

 

Can anyone help me with this problem pls? My background image is a scenery, when I scroll down a long text post, the background image also moved, thus does not look neat. Thanks in advance for helping.

 

Paste this code in Custom CSS
Appearance > Theme Option > Styling Option > Custom CSS

Code:

body{
background-attachment: fixed!important;
}