Complex sentence: Mitty works in the negative assets department, and has never lost a photo in his sixteen years of work at Life magazine, but negative 25, O'Connel's chosen cover photo, is missing.
Split into simpler sentences: Mitty works in the negative assets department, and in his sixteen years of work at Life magazine, has never lost a photo. However, negative 25, O'Connell's chosen cover photo, is missing.