How do I resize the Xul button's image within the binder?

Question

I have a Xul with the following binder:

 <binding id="appicon_bind">
  <content>
   <xul:button xbl:inherits="image=img" />
  </content>
  <handlers>
   <handler event="focus">
           this.height='200px';
           this.image.height='200px';
   </handler>
  <handlers>
 </binding>

Currently, it resizer just the button, but not it's image. How can I resize the button's image either?