Employee's poses as they hold 'Silver Certificate' by Andy Warhol at Sotheby's auction house in London June 8, 2015. The piece has a pre-auction estimate value of 12-18 million pounds ($18.36-27.54 million) and forms part of a private collection of twenty one works inspired by the US dollar which will be auctioned in London on July 1 and 2. [Photo/Agencies]