MLA
Artvue Post Card Co. (New York, N.Y.). H.C. Hopkins Gate at the Alabama Agricultural and Mechanical College. unknown. Robert Langmuir African American Photograph Collection. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/658pc866xp-cor.
APA, 6th edition
Artvue Post Card Co. (New York, N.Y.). (unknown)[H.C. Hopkins Gate at the Alabama Agricultural and Mechanical College]. Robert Langmuir African American Photograph Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/658pc866xp-cor.
Chicago
Artvue Post Card Co. (New York, N.Y.), H.C. Hopkins Gate at the Alabama Agricultural and Mechanical College, unknown, Robert Langmuir African American Photograph Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/658pc866xp-cor.