No problem.
I already added this code, just replace the url with the url of the transparent images.
.extraclass .sf-icon-standard .sf-icon-cont {
background-image: url("http://thebodygarage.com/wordpress/wp-content/uploads/2015/10/icono%20bici.jpg");
background-repeat: no-repeat;
background-size: 80% auto;
}
.extraclass .ss-bike {
display: none;
}
-Rui