Search the Intelligence Database for thumbnails on videos that beat their own channel, by a phrase in the title and by what the picture does: dark background, yellow headline, heavy border, pushed saturation, bright background. Each result carries its multiple.
Type the subject, pick any traits the picture must have, set the floor and search. Every thumbnail shows its multiple against its own channel's normal, and the strip above the grid counts which traits the winners share, so you study what worked, not what is pretty.
A thumbnail that looks good is an opinion; a thumbnail on a video that beat its own channel twice over is a measurement. Searching by image is not here yet; text and measured traits first.