Impossible Figures:
http://en.wikipedia.org/wiki/Impossible_object
Stereograms:
http://en.wikipedia.org/wiki/Stereogram
Anamorphosis (esp. Ambassadors)
Calligram / Arcimboldo:
http://en.wikipedia.org/wiki/Arcimboldo
3D Glasses:
http://en.wikipedia.org/wiki/Anaglyph_image
UV Images (Warhol):
Optical Illusions:
http://en.wikipedia.org/wiki/Category:Optical_illusions
Perceptual Interference (Riley)
Image double-coding (lenin-pollock, duck-or-rabbit, crone-or-maiden)
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