How to use image in global css

Below is my code:

public interface Resources extends ClientBundle
{
@NotStrict
@Source("Global.css")
CssResource css();
}

@Override
public void onModuleLoad()
{
Resources resources = injector.getResources();
resources.css().ensureInjected();
....
}
//Global.css

.rule1{
background:url(images/img1.png);
}
If referring images like,as you can see in rule1,
"background:url(images/img1.png);"(images directory is under war
directory),"img1.png" can't be loaded properly in dev mode,but it got
loaded in web mode.So Question are: 1, how to use image in this
Global.css? 2,Is it possible to use image sprites here?

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