Referred to as "Lenticular" stamps.
Some associated gossip:
acknowledgement authors rcsd:
This is a lenticular lens with very fine spacing - the old 3D
stamps of Bhutan, Umm al Qiwain, etc. use a lenticular lens to come up
with their 3D effects. In the case of the decoder, a few US stamps are
printed with designs that will show up under the lens, if the lens is
held at the correct angle relative to the microlines printed on the
stamp. Maybe the most common of them is the $1 fox definitive? When
you hold the lens against the stamp, a small fox silhouette shows up
within the fox's body. The lens directs your vision to different parts
of the design, depending on the angle you are at relative to the lens.
http://en.wikipedia.org/wiki/Lenticular_lens "Scrambled Indicia" is the copyrighted term for the company that
makes these things. Here's their home page, and an animated GIF showing
the $1 fox stamp image.