CSS Styling with pictures

Hello. 

I wonder if it is possible to do CSS styling with pictures. for example this code for styling a popup that I have: 

.Popup-Style {
 background-color:255,255,255;
 border-color: #000000;
 color: #FFFFFF;
 border-radius: 17px;
 opacity: 0.7;
}

This means that the background will be white of the popup. But what I'm thinking of, is it possible to change the background of plain white to a picture instead? (if you have it uploaded in resources) something like: 

.Popup-Style {
 background-color: /resources/Upload/Awesome_picture.jpg;
 border-color: #000000;
 color: #FFFFFF;
 border-radius: 17px;
 opacity: 0.7;
}