How do I identify a family ancestor in a photograph?

I don't know the person's name. That's what I'm trying to find out.

Ask your oldest relatives and other senior acquaintances who knew your family. Sometimes census information is helpful and if ancestor was in U.S. the 1940 census is now published online and can be accessed free of charge (census.gov). For instance, if you suspect that the ancestor could be a brother or sister of a relative you have some information about and can find the person you are sure of, you may find a brother or sister listed on the census form and you can do further research based on what you find. Be prepared for difficult, time consuming work to find it no matter how you proceed.