I am rather new to all things JavaScript so please bear with me :) I am trying to add ShareThis plugin to the title of FancyBox so that users can share a certain picture. It
I'm guessing that it's not working for this reason.
You are loading the fancy box code when the document is ready using $(document).ready (which is fine). The code that initializes the sharethis elements has probably already run (it's unclear from your code example when you're loading it). So when you create a new set of sharethis elements using $(document).ready, they are not getting processed by the sharethis code.
Try this:
$(document).ready(function() {
$(".wrap").fancybox({
## no change to your code above ##
});
// put your publisher code in below
stLight.options({publisher:'12345'});
});