Worth noting that "snapshot" or "snapshooting" are both terms that originated with guns and gun training, just like "shoot" itself (which actually predates halfway modern guns, perhaps even the 14th century introduction of gunpowder to Europe -- harder to be sure, the language has changed too much since the Thirty Years' War).
A "snapshot" is one taken too quickly to truly aim, depending on reflex, muscle memory, and proprioception for any accuracy it might claim, while "snapshooting" is the act of making snapshots, including training to do so effectively. It's the classic "fast draw" shot of spaghetti westerns (though much less so of the real Wild West), the "from the hip" shot seen in so many gangster movies.
It's easy to see how, after a major war in which tens of thousands, at least, were trained in "shapshooting", the terms might have entered the language in relation to the often poorly thought out, haphazardly composed photos made by rank amateurs, especially in documenting family events like birthdays, holidays, graduations, christenings, weddings, and funerals.