When he was working on the painting prior to its unveiling in 2006, Shanks described it in terms strikingly at odds with his current account. "There are times when I love to play all kinds of complicated games in painting," he told The Philadelphia Inquirer. "But I think this is one case when I need to be fairly straightforward. I'll just try to paint the man, his intelligence, his amiability and his stature, maybe paint him fairly close to humor and try to get it just right."

Perhaps Shanks decided to add the dress later—or perhaps, taking a cue from the dissembling ways of his subject, he opted to hide it. (When you're a portrait painter, there's little chance you'll be impeached for perjury.) But Robert Langdon's fever dreams aside, there's a long tradition of artists hiding symbols and cryptic messages in works commissioned by the rich and famous—sometimes with the commissioner's collusion, and sometimes not.

Take Herbert Abrams's portrait of George H.W. Bush. Behind the president is George P.A. Healey's "The Peacemakers," a painting of Abraham Lincoln meeting with Generals Ulysses Grant and William Sherman and Admiral David Porter in the closing days of the Civil War. Its inclusion is apparently a nod to Bush's international successes—the dissolution of the Soviet Union, and the defeat of Saddam Hussein in the first Gulf War.

Gilbert Stuart's image of George Washington includes symbols of the Roman republic and of American democracy. Aaron Shikler's posthumous portrait of John F. Kennedy famously shows the president looking down, so that his face is obscured. That was Jackie Kennedy's request, he told People: "The only stipulation she made was, 'I don't want him to look the way everybody else makes him look, with the bags under his eyes and that penetrating gaze. I'm tired of that image.'"

Often, however, the symbolism is more straightforward. Mitt Romney's Massachusetts gubernatorial portrait featured both his beloved wife, Ann, and a caduceus-embossed folder to represent the healthcare law he shepherded.

But it's the wry, playful, and pointed hidden messages that are the most fun. The designer Alexander McQueen, while an apprentice on Savile Row, claimed to have chalked an unprintable message into a suit being made for Prince Charles.

Shanks's gesture is in a way much bolder—after all, it would require ripping out the seams of an immaculately tailored suit to detect McQueen's graffiti—and a bit of letdown. How subversive can a hidden meaning be once it has been revealed to the world?

More to the point, is what Shanks did inappropriate? It's not as if the painting was a private commission for the Clintons that Shanks sabotaged: It's a bequest to the nation and to the National Portrait Gallery. (The portrait was commissioned by the gallery and paid for with private donations.) The Lewinsky scandal was a key moment in the Clinton presidency. A consistent two-thirds of Americans have a favorable impression of Bill Clinton, but popular forgiveness doesn't mean the scandal didn't happen and isn't worth recording.