Adding a Background Image to style.css

I’m a bit embarrassed to post this question! I am an experienced web site designer but new to WordPress. A friend asked me to add a link to her FaceBook page in her WordPress site header. She gave me her dashboard username and password.

I logged into her dashbord and added the appropriate code to the header.php file. I uploaded a nice FaceBook image to the image library and it showed up in the Media Library listing. I then went to style.css to add the necessary CSS coding. This included adding a background image within the div that houses her link to the FaceBook page. Everything worked well except I can’t figure out how to specify the url link for the background image! I can’t tell from the dashboard where the uploaded image is stored. What is the directory structure for the background URL?

Say the image is named facebook.jpg. Is it stored in an images folder within the theme folder? Then the url would be simply images/facebook.jpg. This didn’t work. When facebook.jpg was uploaded to the Media Library where did WordPress store it?

Thanks for your help.