MyBundle.css

/* the name "logoImage" must match with the ImageResource's name */

@url logoUrl logoImage;

.logoBackground {
    background-image: logoUrl;
}

MyBundle.java

public interface MyBundle extends ClientBundle {

    interface MyStyle extends CssResource {

        String logoBackground();
    }

    @Source("MyBundle.css")
    MyStyle style();

    @Source("gwt.png")
    ImageResource logoImage();
}