Easy FancyBox does not match href links for images that have a media query appendage.
For example, W3 Total Cache browser cache, when combined with CDN, appends a media query string in the form ?XXXXXX after the filetype.
Easy FancyBox does work properly if W3TC Browser Cache setting is unchecked: Prevent caching of objects after settings change.
I think plugin author should refine the matching routine used by Easy Fancybox. See line ~108+ of easy-fancybox.php
Probably it does not match the filetype if the media query string is appended to the .jpg etc. Need to allow for the W3TC media query string, which is now in the form ?60eab3 -- e.g. file link url ends with blah.png?60eab3