Add custom classes to an image shortcode. (Feature request)

It would be very useful to be able to add a custom class to an image shortcode. So you could disable lightbox for individual images while still having the default on for lightbox pop-ups. I usually have the lightbox on for pages where I want a lot of thumbnail images with a large image pop-up but there will always be the odd image on that page that doesn't need the pop-up on.