NEW YORK, Dec 6 (Reuters) - Pinterest on Tuesday said it reached a deal with Elliott Management, the activist investment firm that has offered up ideas on how to improve operations at the image sharing and social media service company. Marc Steinberg, a portfolio manager at Elliott, will join the board on Dec. 16, Pinterest said. Pinterest CEO Bill Ready, who took the job in June, called the deal with Elliott a "one-of-a-kind agreement" and said the company "appreciates the perspective" Steinberg, Jesse Cohn, a managing partner at Elliott, and others at the firm bring. Pinterest's stock climbed 2.5% in after hours trading on the news that Elliott, one of the company's biggest investors, was joining the board. Elliott, one of the world's busiest activist investing firms, has extensive experience in the tech and social media sector as Cohn held board seats at Twitter and eBay (EBAY.O) and worked closely with Steinberg on those investments.

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