For a story channel, a thumbnail is part of the audience's understanding of the characters. Changing a character's defining features from one episode to another can make a continuing story look like unrelated material.
Write a short visual reference
Record the character's stable features, clothing, and distinctive marks. Separate these from details that can change with the scene, such as pose, weather, and expression. A useful reference is specific enough to catch mistakes without trying to describe every pixel.
Match the current story moment
A thumbnail may use a dramatic scene, but it should fit the episode's actual events. Avoid adding a creature, weapon, or relationship that is absent from the film. Continuity includes the story as well as the character's face or costume.
Check relative scale
If the story pairs a person with a large creature, review their proportions against the established reference. Unexplained size changes can alter the meaning of the scene. Also check identifying details such as eye color, scars, or which wrist carries a mark.
Compare before approving
Place the proposed cover beside the character reference and the preceding episode's thumbnail. List any intentional differences and correct accidental ones. A strong visual identity can accommodate fresh compositions without changing who the characters are. The aim is recognizable storytelling, not repeating an identical image for every upload.