In America in the early 1900s, families would have professional photographs made into postcards. The back was the regular postcard format - left half for a written message and right half for the address, with a spot in the upper corner for the stamp.
I have many of these old photo postcards, but only one that was actually mailed. The rest only identify the people on the front, weren't addressed, but apparently slipped into an envelope with a letter instead.
The two young ladies here are Edyeth "Edith" Anna MORRIS, 6 1/2, and her baby sister, Hazel Velma Morris, b. 10 March 1910 in Lemoore, Kings Co, Calif. They were the youngest children of my great-aunt Eunice Josephine "Aunt Josie" (Cupp) and Samuel Grant MORRIS.
Little Hazel looks to be about 3 months old, so I'm guesstimating this was taken in late May or early June 1910, probably at a studio in Lemoore.
Edith lived to a ripe old 96 (2000), but Hazel passed away in April 1924 at the tender age of 14, the fourth of Sam and Josie's seven children to die in childhood.