I absolutely LOVE the one of the bearded man! How cool that they were discovered behind a painting? You might want to talk to the garage sale seller, and try to find the family who accidently sold them, they might really like to have them back and could possibly tell you who these men were. As they stand now, their history is unknown and you could do these guys a big favor if you were able to reunite them with their family history. It sounds like it could be a wonderful, rewarding adventure.
The style of the mans beard looks very Civil War era to me, which would be typical of a middle-age/older man, keeping what was in style when he was young and is comfortable with. The chair he is sitting in is Eastlake, which was very popular in the end of the 19th century. I think this is going to date to about 1880-ish.