gwt pushButton in UiBinder

I need to create a pushButton(so it's a button with image) in gwt with UiBinder, but I'm not sure how proceed. Here is my ui.xml code:

<g:PushButton ui:field="myPushButton"/>

And in *.java file I defined:
PushButton myPushButton;

How can I add an image to the push button? I've tried the following but won't work:

<g:PushButton ui:field="myPushButton" image="myImage.gif" />

Thanks