According to Walter Borneman in his book Marshall Pass, it is a William H. Jackson photo sold as hand coloured postcards by the H.H.Tammen Co.
Photo in the postcard is reproduced on pg51, also in the Sundance Trails Among The Columbine 1990 on pg90 and on pg30 in CRRA#8 Scenic Line Of The World. It also appears as a sketch in Gunnison by R.H.Dorman on pg17.