Add filters to product images and product thumbnails

Adding filters to both the product images and thumbnails allows 3rd party developers to further customize the HTML output.

Here are a list of things that this filter would allow:

1. Change out prettyPhoto with some other lightbox
2. If we wanted to use Timthumb script or some other imaging script to dynamically generate the images
3. If we wanted to add different REL attributes or even rename REL to DATA-REL to pass validation

Thanks!