From what I’ve gathered from various big time 'tubers I’ve watched for years now, as they slowly trickle tidbits of info about what it’s like working as a YouTuber, and what tools and tips Google gives them to assist: Google/YouTube is the one that recommended using the types of thumbnails commonly employed by uploaders, along with a ton of other things that are ubequitous to the platform (such as the phrase “like, subscribe and ring that bell!”).