as it is now you can't add your own background-size css to allow the pinterest to be retina friendly because there's important set on the background already:
I'm setting my background size in your plugin to 80x80
then putting this css in chrome inspector:
background-size: 40px 40px !important;
which resizes it and fixes the pixel density problem, but then when I add it to my stylesheet it doesn't get applied cause of the importance.