By Terry Baynes

(Reuters) - An appeals court on Monday upheld the convictions and sentences of a Michigan couple charged with trying to extort $680,000 from actor John Stamos.

Allison Coss and Scott Sippola were convicted in 2010 of concocting an elaborate plot to extort money from Stamos by threatening to release pictures of him at a party where there were strippers and cocaine. The defendants were each sentenced to four years in prison.

Coss and Sippola, who were in a relationship at the time, challenged their convictions, claiming the extortion law was vague and therefore unconstitutional. They argued that the statute should only criminalize threats to commit unlawful acts, not just wrongful acts.

Selling the pictures to tabloids may have been wrong, but it was not illegal, they argued, and the law infringed on their commercial speech protected by the First Amendment. But the Cincinnati-based U.S. Court of Appeals for the 6th Circuit rejected their appeal.

"The crime of extortion has never been defined strictly in terms of the lawfulness or unlawfulness of ... the underlying actions," Judge Karen Moore wrote for a unanimous three-judge panel. The threat to sell the photos was wrongful because Coss and Sippola had no right to $680,000 from Stamos, the court concluded.

The appeals court quoted the trial judge, who said that presenting the plot as a legitimate business deal was "to put it mildly, somewhat ludicrous."

Sarah Henderson, a lawyer for the defendants, did not immediately respond to a request for comment.

The U.S. Attorney's Office in Grand Rapids, Michigan, did not immediately provide comment.

Coss, who worked at a northern Michigan night club, met Stamos in Florida in 2004 when she was 17 and on a trip to Disney World. The two continued to correspond via email for several years.

Stamos contacted the FBI in Nashville, Tennessee in November 2009 after receiving emails from someone claiming to have found incriminating pictures from 2004. Prosecutors said Coss and Sippola had sent the emails and that the damaging pictures did not exist. The two also sent emails to Stamos posing as a woman who claimed to have been impregnated by him, prosecutors said.

Stamos is best known for his role as Jesse in the 1990s sitcom "Full House," where he played opposite then-child stars Mary-Kate and Ashley Olsen.

(Reporting By Terry Baynes; Editing by Steve Orlofsky)