Logo in Menue

Hi there,
is it possible to add a logo to the menu instead of text?
I found an tutorial showing this for a mega menu item, but this doesn´t seem to work for a regular menu.

Here´s the page:
http://wordpress.herr-pimock.de

My client want´s the Facebook Menue item to be the fb logo.

( I know that i could add social icons to the header, but as my client only uses FB, the little “f” at the top of the header would not really stick out.)

Thanks a lot
Marco