This is believed to be a photograph of John Adam Saltzer and wife, Emma Savilla [Rothermel] Saltzer, and daughter, Mabel Alvena [Saltzer] Daniel (and dog!) of Lykens Township, Dauphin County, Pennsylvania.
Mabel was born on 1 January 1899, so dating this photograph would rely on her age at the time it was taken. She appears to be about 7 years old, thus dating the photo at around 1906.
The Saltzer’s had another child, William A. Saltzer, who was born in 1891 and died in 1892.
Corrections and additional information should be added as comments to this post.