How to change size of image for toolbarbutton on the fly

Sometimes you need to use for toolbarbutton an image which is returned by some third-party service. And it's size can be much greater than you need:

<toolbarbutton id="my-btn-id" label="My Button" image="http://somesite.com/images/img.png"/>

In this case you need to add one line of code in CSS file:

#my-btn-id .toolbarbutton-icon {
  width: 16px;
  height: 16px;
} 

This simple trick will allow you to get toolbarbutton image with necessary size.