This sounds like a great idea to add to your book. One thing I'm thinking about is that as a buyer/owner of your book I think I would like to have the ability to enjoy that image a tad longer. Maybe have the opportunity to see the image fade over the course of a day or evening, or maybe have the opportunity to revisit the image in it's faded state over the course of a week or so, enjoying it for say 15 minutes and then putting it back in the envelope.
This way people would have more opportunity to enjoy the process, and an accidental opening of the envelope does not immediately destroy the image.
Anyway, just my thoughts. As I said, a great idea none the less.