Thumbnail for Vimeo or Youtube

Hi I am trying to put a thumbnail that links to a vimeo (or Youtube) movie in an Easyfancybox pop up.
The embed works fine as per the FAQ example:

text or thumbnail

However when I put a link to an image/thumbnail (copied from my media gallery) all I get is text for the link and no image.

I must be missing something here. Can anyone put me out of my misery?

https://www.ads-software.com/plugins/easy-fancybox/