Is your image on the same folder/directory as your css file? If so, your image url is correct. Otherwise, it's not.
If by any chance your folder structure is like so...
webpage
-index.html
-css
- - style.css
- images
- - background.png
then to reference the image on your css file you should use the following path:
../images/background.png
So that would be background: url('../images/background.png');
The logic is simple: Go up one folder by typing "../" (as many times as you need). Go down one folder by specifying the folder you wish to go down to.