Upload 2–3 thumbnail options and your video title. AI scores each image on small-size readability, contrast, faces / emotion, text of 4 words or fewer, match with the title, and curiosity, then picks the winning image with reasons and fixes. 2 credits per run.
This is the thumbnail size in suggested-video lists on phones. Images whose text is still readable and whose subject is still recognizable at this size usually get clicked more.