That is a really cool photo!
A little research would put my guess at the 1930's (might be late 20's), based on the vehicle parked at the pavilion and the price of gas on the pump sign.
Gas prices by year since 1919.
http://www1.eere.energy.gov/vehicles...t_fotw364.html
The unknown is how old (read how long the owners had them) those vehicles are in the photo.