Thank you for the kind words. :)

I've been down with a bloody urinary tract infection, so have to recover myself before fixing the computer.  Perhaps some of these approaches can spur more entries?

Remember those hidden images posters and puzzles?  Some types of play are fun, such as this teddy bear in the clouds:

http://www.planetperplex.com/en/pareidolia.html

And others, such as "magic eye" floating images are just fiendish.

http://www.planetperplex.com/en/magic_eye.html

Anamorphosis, is mostly twisted images (look at some Renaissance paintings through grooves in the frames to see (OMG) a huge skull, or other such trickery.  Or place a metallic tube into the middle of a drawing to see the unbent original.  I think of it as a gambit, but many masters liked playing with their art.

Anyway, the Planet Perplex site has lots of fun, different things to do when making an image:

http://www.planetperplex.com/en/index.html

Silhouettes hiding other images?

Catasterism was unintentional: viewers pointed out the "giant parrot head" because they saw it from across the room, whereas my POV was inches away.

http://www.renderosity.com/mod/gallery/index.php?image_id=1037133

And silhouettes lead to...

http://en.wikipedia.org/wiki/Rorschach_inkblot_test

(Years ago, this test was given to me.  One of my responses was "trireme", and I had to explain to  the pysychologist that the Greeks invented a vastly superior warship with 3 banks of oars and lots of pointy bits for ramming enemies.  Intelligent, widely reading, artists do very badly on tests like this.)

Maybe one of these ideas is a starting point?

Carolly