[Plugin: FancyBox] Turn on for certain images in loop

I’m using FancyBox to display full images of each work in my wordpress portfolio page, but only 2 need to show full images and the others just display the image that shows on pageload. My question is can I somehow add a class to these 2 items in the loop so only these are affected by fancybox. I’m using custome post types for each portfolio item.

Any help is appreciated.