Add Fancybox to images link to media file, separate single images from galleries. This goes before the closing body tag and presumes that Fancybox is enqueued.
jQuery(document).ready(function() {
jQuery( "a[href$='.jpg'], a[href$='.jpeg'], a[href$='.png'], a[href$='.gif']" ).fancybox({
openEffect : 'elastic'
});
});
// Images inside <div class="gallery"> as in WordPress
jQuery(document).ready(function() {
jQuery( ".gallery a[href$='.jpg'], .gallery a[href$='.jpeg'], .gallery a[href$='.png'], .gallery a[href$='.gif']" ).attr( 'rel', 'gallery' ).fancybox({
openEffect : 'elastic'
});
});