Dollars, euros and pounds are easy to visualize: They're physical objects. Being digital, bitcoins aren't tangible — which has become a boon for George Frey.

"I've sort of cornered the market on bitcoin photos," the Provo, Utah–based photographer says in the November issue of Bloomberg Markets magazine.

More from the November issue of Bloomberg Markets:

Indeed, dozens of media outlets, including Bloomberg News, have published Frey's pictures. The image used here — dated April 26, 2013 — happens to rank as the most popular bitcoin photograph on Gettyimages.com. But if a bitcoin isn't a physical object, what exactly are we looking at in Frey's photographs?

They're called Casascius bitcoins, and they were minted, in a variety of metals, by software engineer Mike Caldwell at his home in Sandy, Utah. An early bitcoin adopter, Caldwell wanted to help popularize the cryptocurrency — but he, too, grappled with its intangibility.

"No one is going to get this if I can't show them something," Caldwell remembers thinking.

So in September 2011, he began making physical coins as vessels. Inside of each, he embedded a piece of paper that contained a bitcoin private key, which he protected with a tamper-resistant hologram sticker. As for the word casascius, it was half acronym (derived from the phrase "call a spade a spade"), half Latin-sounding suffix ("cius").

Bitcoins

As of Nov. 27, 2013, however, Caldwell no longer includes digital bitcoins in his physical bitcoins. A letter from the U.S. Treasury Department's Financial Crimes Enforcement Network, or FinCEN, stopped him in his tracks: FinCEN considered his activity to be money transmitting and informed him that he lacked the necessary license.

Since then, Caldwell has sold only aluminum promo coins via his website, Casascius.com. A bag of 500 costs 0.39 bitcoin — about $150, as of yesterday. All told, Caldwell minted about 60,000 Casascius bitcoins. Bitcoin enthusiasts consider them collectibles — especially the earliest ones, which have fetched as much as $2,500 each on EBay because of a typo in the hologram.

Not that much of this matters to Frey, whose photos continue to be downloaded via Getty. Frey says he's made more than $10,000 off the images, which he shot at Caldwell's home.

Unlike Caldwell, he accepts only U.S. currency for his work.

To contact the reporter on this story: Joel Weber in New York at [email protected]

To contact the editors responsible for this story: Daniel Ferrara at [email protected]