I'll try one more time to get this message posted -- third time's a charm, I'm told.
Besides the column and the lion mentioned above by jmd, some modern decks represent Strength with a snake. I believe this is a reference to the serpent Kundalini, the Hindu metaphor for the life force, who coils around the base of the spine, and whose activation is the object of the various forms of chakra meditation.
But I can't help but think of Strength as the lady opening the jaws of the lion (or is she forcing them shut?), as the image has been codified and enshrined in the Marseilles decks. There was that one image of Hercules killing the Nemeian lion, which Kaz was nice enough to post, in the Visconti-Sforza deck, but that picture was, I think, way too violent to ever catch on. It's one of six cards that was painted by a different (and later) artist than the one who was originally commissioned to produce the deck. Stuart Kaplan has suggested that the card's human figure is a portrait of Francesco Sforza, (Encyclopedia II, p. 101) and I can imagine a second-string artist trying to make points with the patron by portraying him as Hercules. Francesco Sforza didn't do badly for himself, considering that he began as a mercenary warrior and ended up the Duke of Milan.
Most interpretations of this card that I've seen follow the thesis that the picture represents people's need to control and repress their dangerous or disruptive feelings and impulses. However, following the clue offered by the card's title -- La Force, a somewhat inaccurate translation of La Fortezza -- I think it shows courage in the face of danger, and the ability to persevere through hardships. Tom Tadfor Little in his "The History of the Strength (Fortitude) Card (in "The Histories of the Trump Cards" at
www.tarothermit.com) puts it this way:
"In Aristotle's thought, Fortitude represents moderation in our responses to fear and pain. The virtuous person neither seeks out danger nor avoids it at all cost, but instead courageously faces danger when necessary to achieve a higher goal. To endure the difficult, to stand firm against opposition, and to master one's fears--these are the tasks of Fortitude. Modern interpretations of the card sometimes stress the idea of the spirit controlling the urges and passions of the body. In the classical understanding of the virtues, this is a concept more at home with Temperance than with Fortitude."
Following Waite's re-ordering of the trumps, I put Strength at position VIII in my own deck, and now I'm kind of sorry I did. Participating in this forum has caused me to gain considerable reverence for the tradition codified and enshrined in the Marseilles decks, and I will probably always think of the Marseilles sequence as "the correct one." This in spite of the fact that the virtues have always been the cards which exhibited the most fluctuation in the trump sequence, because finding the "proper" place for them has been a perennial problem. For the first couple of centuries of Italian tarot they shifted all over the place in the deck, from time to time and from place to place. Interestingly, the virtues in the Marseilles sequence (which is based on an earlier Milanese sequence) are exactly three spaces apart, at VIII -- XI -- XIIII, which mostly makes sense, although it still leaves the problem of Temperance being placed between Death and the Devil. More about that later.
Also interesting is the fact that Greek depictions of Hercules killing the Nemeian lion look more similar to the Marseilles decks' picture of La Force than they do to the Visconti-Sforza's Hercules, since the demigod strangled the animal, rather than beating him with his famous club. Check out the pictures on these red-figure Attic
vases:
http://www.theoi.com/Tartaros/Nemeian.html
It is the hide of the Nemeian lion that Hercules is generally seen wearing as a cape. Maybe that's the plan of the lady on the Marseilles Strength card -- she's planning to make herself a fur coat.