I think you're right about the date, the Oldsmobile Six in the billboard is $750, and here's an ad for a 1927 that was a couple hundred more, but looks about the same. I'd guess mid-20s for the picture. The tires on the bike are also way too bulky for a 1915 machine, so I'd guess the bike is early to mid-20s as well. The headlight is positioned right for something like a 1925 Harley Davidson.
check out the side view of this 1925 Harley:
it even has the same diamond design on it.