This real photo postcard (RPPC) is indeed an actual photograph, with postcard markings stamped on the back. The stampbox indicates that the paper was made by a company called AZO and, based on the stampbox’s particular design, the date of manufacture can be narrowed down to sometime between 1904 and 1918. Unfortunately, it is hard to narrow it down any further with any degree of certainty.
The detail at right of the three people in the photograph is exactly a one inch square. After putting this detail through several filters and enhancements, I am reasonably sure that the folks pictured are African-Americans. Click on the image for yourself to see a larger version.