A/B testing, which is the point of the paid TubeBuddy tier. Running two thumbnails against each other on the same video gives a click-through number rather than an opinion, and that number is what renews a retainer. Without it you are arguing about taste.

Less than you would think. The rules are simple and repeatable: one clear subject, a face or a focal point, three or four words maximum, and high contrast. The commonest mistake is designing at full size and never checking what it looks like at phone-thumbnail size.

Because viewers judge them together, and a great thumbnail with a vague title still gets skipped. Offering three title options per video costs you very little and makes the service noticeably more valuable than one that hands over an image.

One that posts often enough for the work to be a habit, weekly at least, and that has a niche you can study. Channels that upload occasionally will never see enough data to know whether your work helped, which makes them hard to keep as clients.