By Terry Baynes

(Reuters) - A U.S. appeals court on Monday upheld the convictions and sentences of a Michigan couple who tried to extort $680,000 from "Full House" 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 challenged their convictions, claiming the extortion law was vague and therefore unconstitutional. Selling the pictures to tabloids may have been wrong, but it was not illegal, they argued.

But the Cincinnati-based U.S. Court of Appeals for the 6th Circuit rejected their appeal. The threat to sell the photos was wrongful because Coss and Sippola had no right to $680,000 from Stamos, the court concluded.

Coss, who worked at a 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 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)