The Library of Congress site for this collection states that the S.M. Prokudin-Gorskii photographs are probably in the public domain. They were taken before 1923 and bought by the Library from Gorskii’s heirs in France in the 1940s. But no clear policy about their copyright was addressed at that time. The LOC suggested contacting the grandson in France. I did. He kindly granted me permission to use the images on my cards (though he preferred not on coffee mugs).
“The Sergei Mikhailovich Prokudin-Gorskii Collection features color photographic surveys of the vast Russian Empire made between ca. 1905 and 1915. Frequent subjects among the 2,607 distinct images include people, religious architecture, historic sites, industry and agriculture, public works construction, scenes along water and railway transportation routes, and views of villages and cities. An active photographer and scientist, Prokudin-Gorskii (1863-1944) undertook most of his ambitious color documentary project from 1909 to 1915. The Library of Congress purchased the collection from the photographer’s sons in 1948.”