snake symbolism...
Some people have already hit on snakes symbolizing change and transformation because they shed their skins and emerge anew (toads and frogs have symbolized similar things in world folklore because they also reemerge renewed after winter). I'd like to add that snakes are often agents of change--in Greek myth, the prophet Teireseis was changed from a man to a woman after encountering two snakes coupling. He killed the female snake, and became a woman for seven years. Then he encountered two snakes going at it again, and this time he killed the male of the two, and was transformed back into a man. This led to his gift of prophecy because Zeus and Hera were squabbling over who got more pleasure from sex, men or women, and so they found Teireseis to settle it since he'd experienced life as a man and as a woman. Hera had been denying that women got more pleasure from sex, and when Tireseis affirmed it, she blinded him out of rage. Then Zeus gave Teireseis the gift of prophecy to compensate him (I can just imagine Zeus saying "sorry dude, my wife's kinda bitchy sometime, bummer about your sight, um, have some foresight?").
Another example of a serpent instituting change comes from certain aboriginal peoples of Australia. In the Dreaming Time, the Rainbow Serpent came upon two women who were dancing. Their dancing caused their menstruation, and the blood attracted the snake, who devoured them but them regurgitated them, and from them the first people were born.
Also keep in mind the French legend about Melusine, temptress and enchantress who assumed a serpentine form and was discovered by her human husband.
Lest these folkloric examples sound totally random, keep in mind that British folklorists from William Butler Yeats to Alfred Nutt have been in touch with influential members of the Golden Dawn and other occult orders. I think some of these ideas may have been circulating in the intellectual climate when Crowley was designing his deck, which could be pure speculation on my part, but hey, I enjoy discussing snake symbolism. (for further reading, see a book called _Metamorphosis: The Dynamics of Symbolism in European Fairy Tales_ by Francisco Vaz da Silva, a brilliant Portuguese symbolic anthropologist)