I noticed that the website embeds a lot of the pictures in a flash application. Search engine crawlers don't understand flash and don't understand flash, and they don't know that there are images inside.
Not sure if Zenfolio has a way to work around this, but it could be that the crawlers don't even know you have all these great images on your website. It should find the thumbnails on the index pages like http://rwilliamsimaging.com/p534263534
, but I am not sure if they see the full size version.