GWT ImageResource accessed within a CSS property?

The Google/GWT example of creating an ImageResource is understood:

interface MyResources extends ClientBundle {
  @Source("image.png")
  ImageResource image();

  @Source("my.css");
  CssResource css();
}

@sprite .myImage {
  gwt-image: 'image';
}

I understand how to use ImageResources and apply style names, however...

In my application, I have multiple themes that are applied to various widgets using CSS and deferred binding. So I have defined a CSS rule ('background') that I would like to use the .myImage class, but it does not do anything:

background {
    background-attachment: fixed;
    background-image: .myImage;  //??  This is the question!
    background-size: contain
}

What is the syntax for using the .myImage class within the 'background' CSS property? It seems I should be able to specify the .myImage class as the argument for background-image.

Edit: Did some more research and found the correct syntax to do this using a DataResource.

MyClientBundle extends ClientBundle {

    //Background Image
    @Source("resources/background.png")
    DataResource backgroundImage();

}

(mypanel.css)

@url background backgroundImage;

.myPanel {
    border-radius: 0px;
    background-color:#ffffff;
    opacity:0.6;
    background-image: background;
}