0-The Fool-Harry Potter
Traditionally the symbol of courage, innocence, faith, and adventure, the Fool is also the Seeker -- the prime mover on the path.
1-The Magician-Albus Dumbledore
For obvious reasons. Of any of the wizards we've seen, he's the one who most embodies the balance of the elements. He is prepared, dedicated, committed to his spiritual path.
2-The High Priestess-Minerva McGonagall
A figure of formidable power, focus and mystery, concerned with mystical matters, balanced.
3-The Empress-Molly Weasley
This is the most maternal card in the deck, showing a beautiful, mature woman sitting in the midst of fertility symbols in most decks. Molly is the closest thing to a mother figure Harry has.
4-The Emperor-Rubeus Hagrid
An older man often surrounded by animals, sometimes holding a shepherd's crook. It tends to be a very traditional image of masculinity -- oversized, paternalistic, yet benevolent.
5-The Heirophant-Lucius Malfoy
This is the figure that's traditionally portrayed as a Pope -- the formal religious leader rather than the spiritual guide, the one invested with earthly powers. Patriarchal but not benevolent, though sometimes a trained healer. I could make a case for either Fudge or Filch for this card, too.
6-The Lovers-James and Lily Potter
I know, it's wimping out to pick a canonical heterosexual pair, but I wanted them in the Majors, they're no longer living, they're almost archetypes rather than individuals of whom the Fool has intimate knowledge. I'd put them here, on the card that often shows Adam and Eve, the parents.
7-The Chariot-The Weasleys' Enchanted Car
Moves between the Muggle world and places visible only to wizards. This card is associated with pride -- sometimes hubris -- and when Harry and Ron take it upon themselves to fly it, they are arguably guilty of those things as well.
The traditional image on this card is a woman holding open the jaws of a lion. It's a card of passion and commitment as well as power and physical prowess.
9-The Hermit-Severus Snape
This is a card of contemplation, introspection and wisdom in its positive aspects and of isolation, secrecy and disguise in its less positive aspects, all of which suit the potions master perfectly.
10-The Wheel of Fortune-Sibyll Trelawney and her Crystal Ball
The Wheel itself is often portrayed surrounded by symbols of power -- a sword-wielding sphinx, a serpent, a book -- and astrological symbols. The implication is that what goes around comes around, and that destiny and fortune can't fight prophecy and the elements.
Traditionally the image is the balanced scales and sword being held by a blindfolded figure, but the meaning has as much to do with balance and the punishment for transgression and overreaching as with anything legalistic. I thought about reversing this and the Wheel, since the Time-Turner obviously works more like a wheel of fortune, but Trelawney isn't really blind justice with a sword in her hand, she's the receptacle of the fates speaking through her.
12-The Hanged Man-Sirius Black
This card represents an end to old patterns and a transformation of life. The figure has pretty evident parallels with Christ -- he's often portrayed hanging upside-down, suspended by one foot from a tree - yet he appears relaxed, pensive, not at all terrified or in pain. It's a card of trial and sacrifice, altered perspectives and new insights. And sometimes it represents being scapegoated, taking an unpopular position, being isolated.
13-Death-The Dark Mark
Often a skeleton holding a scythe or just a skull dominates this card. It doesn't usually mean literal death, but an end to old cycles, old beliefs. I imagine anyone who takes the Dark Mark has to cut off a lot of his or her past to commit to Voldemort.
The central figure in this card is frequently an angelic figure with a halo, pouring water from one chalice into another without losing any drops. The image suggests that which completes, moderates or makes sufficient the querent (the Fool). It also suggests inner qualities of balance, and satisfaction with one's assets.
15-The Devil-Tom Riddle/Voldemort
Classically, a bearded male figure with horns, pointed ears, bat-wings and talons sitting over an inverted pentacle. He's the inverse of the Heirophant, and the Lovers are sometimes illustrated as his prisoners (Adam and Eve after the fall). I might choose Riddle over Voldemort because it's also a card of carnality and passion, not necessarily evil; uncontrolled desire, being trapped by lust or greed.
16-The Tower-Gilderoy Lockhart
The Tower is almost always portrayed as shattered or falling so I would NOT use Gryffindor Tower for this. It's the Tower of Babel, the card of arrogance and overreaching oneself until everything comes crashing down.
17-The Star-Luna Lovegood
This card often depicts a young woman (naked, but you can probably work around that, heh) kneeling beside a pool, pouring water onto the flowering ground while above her head, a large star dominates the sky. It suggests brilliance, bright prospects, self-sufficiency, benevolence.
18-The Moon-Remus Lupin
I know it seems rather odd to put him on the card of the thing he fears the most, but the moon in the Tarot is associated with renewal and regeneration as well as secrets and mysteries. It concerns hidden emotions, shedding light on unexpected paths, revealing what is hidden -- both the good and the bad -- perfect for a Defense Against the Dark Arts teacher as well as a werewolf.
19-The Sun-Draco Malfoy
In Rider-Waite based decks this card shows a beautiful child riding in a walled field with an unsmiling, brilliant sun above. The child might be the Son as well as the sun, which looks stern, as if suggesting that happiness is contingent upon remaining within the walls, obeying the rules.
This card is not judgment as in justice, but the Last Judgment when the dead will rise.
21-The World-The Sphere of the Prophecy
The World is often portrayed not by the planet but by a young naked woman in the center of a garland, sometimes suspended between heaven and earth, with symbols of the elements around her. It can represent so many things: the union of heart, mind, body and soul; carnality, intellect, emotion and spirit; wit, determination, stubbornness and strength; past, present, future, full circle; Gryffindor, Hufflepuff, Ravenclaw and Slytherin; fire, earth, air and water. At the center is the individual or object that can integrate all these aspects. There are a million ways you could go with this one, from making Ginny the girl in the middle if you believe that Harry is destined for her to putting the Mirror of Erised there is you think it's all a grand illusion!