Oh, I would note, if you're at all comfortable with editing PHP, this is technically supported in the current plugin. Just not in the dashboard itself as this is an advanced feature. Open the source and search for "Add your own style":
/* Add your own style here, like this:
, 'custom' => array('img'=>'http://example.com/button.gif', 'w'=>16, 'h'=>16) */
Just change the URL to your image, w from 16 to your image's width, and h from 16 to your image's height. Then move the close comment (*/) to right after "this:". So you'd have
/* Add your own style here, like this: */
, 'custom' => array('img'=>'http://example.com/mybutton.gif', 'w'=>160, 'h'=>24)
Best,
Matt