By Jennifer Ablan

NEW YORK (Reuters) - The treasure that a New York City woman saved from the trash -- a stolen masterpiece by Mexican artist Rufino Tamayo -- is expected to go for as much as $1 million at auction next month.

Elizabeth Gibson will get a $15,000 reward for returning "Tres Personajes" ("Three People") to its rightful owners, as well as an undisclosed percentage of the auction price.

Nearly four years ago Gibson was on her way to coffee when she spotted the painting among garbage bags set out for morning collection in her Upper West Side neighborhood.

She walked by it at first but said she "immediately knew I had to go back. I knew I had to take it!"

"It was a huge, powerful and beautiful painting and I said to myself, 'It is wrong to be in the garbage,'" Gibson told Reuters on Tuesday.

Gibson, who had hung up the painting after her discovery, said she learned of its worth when research led her to the Web site of "Antiques Roadshow FYI," a companion program to the PBS show "Antiques Roadshow."

Sotheby's expert August Uribe had featured the painting on "Antiques Roadshow FYI" after it was stolen more than 20 years ago.

"Tres Personajes" was returned to its owners, who wish to remain anonymous, and is expected to fetch between $750,000 and $1 million when Sotheby's auctions it on Nov. 20.

If Gibson had not acted so quickly after spying the painting, it might have been lost forever. A doorman nearby later told her that garbage collectors had come along just 20 minutes after she had left with her new finding.