People were straight clowning around back in the day. I bet a couple of guys were hanging out, bored (since there weren't any video games or social media), when one of them was like, "say, Phillip, why don't we take a photograph of us riding a bicycle with a skeleton so that people may gaze upon it as if the skeleton were a living ghoul!" and then they all yelled, "chip-chip-cheerio!" I assume that's how most interactions happened around this time.
Taking a photo was so difficult back then that you might as well go really big with it. If you're not going all out with props and concept then you just wasted one of those exploding lightbulbs, as well as an hour of everyone's time. I'm sure skeletons were plentiful too since there were less laws on who could sell and handle a human skeleton.