LONDON, England (CNN) -- A picture of an overweight woman lying naked on a couch, painted by British artist Lucian Freud, set a record Tuesday night for the most money paid for a painting by a living artist.

"Benefits Supervisor Sleeping" depicts Sue Tilley lying on a worn-out couch.

The 1995 life-size work, "Benefits Supervisor Sleeping," fetched $33.6 million during bidding at Christie's auction house in New York. The previous record was for "Hanging Heart," a painting by Jeff Koonz that sold for $23.5 million, said Rik Pike, a spokesman for Christie's.

"Benefits Supervisor Sleeping" depicts Sue Tilley, a manager of a government-run job center in London, lying on her side on a worn-out couch with nothing to hide her folds of flesh.

Christie's calls it a "bold and imposing example of the stark power of Lucian Freud's realism," depicting "the forceful and undeniable physical presence of people and things."

Tilley, 51, said she was initially embarrassed to pose naked for the artist, but they soon grew comfortable in the studio -- so comfortable, in fact, that she confessed to falling asleep while posing.

"I didn't mind if he noticed," she said.

The painting challenges modern notions of beauty and elicits a reaction from everyone who sees it. That may have been precisely the aim of Freud, who told London's Tate Gallery in 2002 that he wanted his paintings to "astonish, disturb, seduce, convince."

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Though some regard the painting as shocking -- ugly, even -- that is also the appeal for collectors, said Michael Hall, editor of Apollo Magazine in London.

"There's a reaction against art that's regarded as too pretty," he said.

Hall said he thinks a more conventionally beautiful painting would not be able to fetch such a large amount.

"It's the sort of thing that everyone immediately wants to voice an opinion about," he said of the painting. "It challenges conventional taste ... and people do find that rather exciting and interesting to talk about."

Collectors may also view this as a rare chance to buy something by a prolific artist painted at the peak of his work, he said.

Freud, 85, has been described as Britain's greatest living realist painter. He is the grandson of psychoanalyst Sigmund Freud and came to London from Germany when he was a child.

With Tilley, Freud said he was "very aware of all kinds of spectacular things to do with her size, like amazing craters and things one's never seen before," according to the 2002 interview with the Tate. He added, "I have perhaps a predilection towards people of unusual or strange proportions, which I don't want to over-indulge."

Freud painted the portrait of Tilley over nine months in 1995. Tilley said she posed for eight hours a day, two or three days a week.

She had been introduced to the artist through a mutual friend, Australian performance artist Leigh Bowery, who also posed for Freud. It was Bowery's idea for Tilley to pose for Freud, so he arranged a meeting.

Tilley knew the meeting was more of an interview for the job of Freud's muse, and she didn't find out until later -- through Bowery -- that she'd gotten the job, she said.

"Lucian just said to Leigh, 'Oh, tell Sue she can start next week,' " Tilley said.

Tilley still works full-time at the job center in London's West End and calls her newfound fame "a bit bizarre." She laughs as she describes how she now has to arrange her schedule to accommodate media interviews.

She said she's excited to find out how much the painting will sell for, but knowing that it could set a record is "a bit scary."

"It's hard to put your head around it, really," she said. "But it's all good."

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