Custom Links dont show my uploaded image

Hi there.

Trying to add social media icons to my may nav and only get a transparent square. Im following this tutorial “https://www.youtube.com/watch?feature=player_embedded&v=KLT0pXkvcSE”. The link works fine but the image is not displayed.

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

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

Thanks.