A rare photo of legendary outlaw Billy the Kid, purchased for less than a subway ride, could now fetch $5 million.

The outlaw, whose real name was William Henry McCarty Jr., was playing croquet with his gang of Lincoln County Regulators in late summer 1878 when this 4-by-5-inch tintype image was captured, according to Americana company Kagin’s, which is negotiating the sale of the photo.

The picture was purchased by collector Randy Guijarro for $2 as part of a miscellaneous lot at a Fresno junk shop in 2010, according to Kagin’s and the National Geographic Channel.

The image’s backstory will be the subject of a Nat Geo Channel special Sunday at 9 p.m., “Billy the Kid: New Evidence,” narrated by Kevin Costner.

“When we first saw the photograph, we were understandably skeptical — an original Billy the Kid photo is the Holy Grail of Western Americana,” said Kagin’s numismatist David McCarthy.

“We had to be sure we could answer and verify where, when, how and why this photograph was taken. Simple resemblance is not enough in a case like this — a team of experts had to be assembled to address each and every detail in the photo to insure that nothing was out of place.”

Kagin’s based its price estimate on the sale of the only other known Kid snap — which went for $2.3 million in 2010.