A “transformation deck” is a pack of cards where the front of each card has been transformed: where an illustrator has taken the standard front – four diamonds, or three spades, or whatever else – and drawn a picture around it: turning hearts into tulips, say, or clubs into faces.
It sounds like a nice but slightly eccentric thing to do; but it turns out, people have done it a lot, starting in 1803 and never stopping since.
For more information, try:
- This page showing 24 different transformations of the 4 of Diamonds
- This history of transformation decks
- The Wikipedia page (source of two of the cards above)
- A full deck on Peter Endebrock’s playing-card pages (source of the fish above)
Transformations range from straightforward to oblique, staid to mildly pornographic; but the primary design rule seems to be “when in doubt, turn the pips into hats”.