I tried surfing a bit about Senitypes and it's incredibly difficult to actually find anything that fully explains what it is! But I did find this snippet , about Cinderalla of all movies;
"The next goodie inside this package is the Cinderella Senitype (that is, a film frame reproduction), which is enclosed in its own (removable) plastic wrapping. The cel is surrounded by a piece of cardboard which shows the Special Edition logo and the glass slipper, looking very classy as on the main box. The frame itself depicts a shot from Cinderella's dress transformation (a favorite scene for many fans of the film, including reportedly Walt Disney). Each person may receive a different still from the scene, however, I got one right as she is looking down in awe, with the fairy dust just disappearing (No. 139713). The back of the cardboard is white and describes the scanning and reproduction process used to create the Senitype."
Full link at http://www.ultimatedisney.com/cinderella-giftset.html
The Holy Grail Senitype I have is of the six Python members looking up towards the camera, whcih I presume to be the God scene, or possibly the Rude Frenchman scene. Assuming that film is shot at (I think) 25 frames pers second then you can figure out that a 4 second scene will produce (duh) 100 film "cels" (shouldn't that be "cells"?), all looking somewhat similar.