A thumbnail is a video's preview image, and together with the title it decides whether anyone will play the video. It is a pair that has to work together – the title should finish saying what the image hints at, not repeat it. The thumbnail must be legible at small size on mobile, so it can only carry one strong element, very short text and contrasting colours; detailed close-ups and long captions get lost. The title should be specific and state what the viewer will gain, not a generic name for the topic. Test both, and for videos with a longer lifespan it is worth swapping the thumbnail after some time and comparing click-through rate. A misleading pair will increase clicks and destroy both retention and trust. For a series of videos, it is worth keeping a consistent visual style for the thumbnails, so the viewer recognises your videos even before reading the title.
See also: Hook in the first seconds, Click-through rate optimisation, Viewer retention.
