Custom Links dont show blank box

Hi there.

Trying to add social media icons to my may nav and only get a transparent square. Im following this tutorial “http://www.youtube.com/watch?feature=player_embedded&v=KLT0pXkvcSE”

Iv done all what there is to do but i just get no image :/

Here is my site.
http://www.promotethebass.co.uk/

Thanks.