I needed to disable fancybox on certain images and after finding that this isn't possible with the default plugin, I dug into the code and patched it (version 3.0.1).
Find:
// Supported file extensions
var thumbnails = jQuery("a:has(img)").filter( function() { return /(jpe?g|png|gif|bmp)$/i.test(jQuery(this).attr('href')) });
Add after jQuery("a:has(img)"):
.not(".nolightbox")
Final result:
// Supported file extensions
var thumbnails = jQuery("a:has(img)").not(".nolightbox").filter( function() { return /(jpe?g|png|gif|bmp)$/i.test(jQuery(this).attr('href')) });
Now, in your post's HTML, add class='nolightbox' and optionally target='_new' to the <a> element, and it will no longer be fancyboxed.
Enjoy!