Antique dealer Robert Lloyd is the world's number one John Gilroy dealer. The name might not be familiar, but you've definitely seen his work—even though it might not have registered with you as art. Gilroy is the man behind the Guinness ads which grace pubs around the world.

Over the course of his career, the English artist, who died in 1985, created thousands of ads for the rich, dark beer. Though Gilroy's posters have been widely reproduced, his original artwork went into archival storage at S.H. Benson. Benson is the British advertising agency that represented Guinness from 1929 through the late 1960s and where the posters lay forgotten for almost 50 years.

Some of the paintings caught Lloyd's eye when they unexpectedly turned up at a Florida antiques show. Though he wasn't familiar with Gilroy's work, after 30 years in the business, Lloyd recognized that he had come across something rare and valuable. He snapped up some 400 paintings, which he now sells for between $9,500 and $40,000. In total, Lloyd may have discovered $2 million worth of Guinness artwork.

Though it may seem strange that such iconic images were lost for so long, there is a ready explanation: "When you lose your biggest client, you certainly don't really care about what you did for them in the past," Lloyd explained to CNN. Today, he does a brisk business in the paintings, finding that first-time collectors often become repeat customers, eager to add another classic Guinness ad to their collection.

A particularly popular series sees Gilroy insert a pint of Guinness into famous artworks, such as the Mona Lisa, or Gustave Doré's painting of Moses unveiling the Ten Commandments, now swapped out for a keg of Ireland's favorite brew. "It's everything that art should be," said Lloyd. "It makes you smile."