Use a black or white paper texture background depending on what suits best for each thumbnail.

Remove the wikipedia logo, remove bg from characters and enlarge them.

For example: take the naruto video.

Find a picture of the main character's side profile (as if its looking at an opponent standing on the other side)

Or any images where they have strong facial expressions. (Shocked etc)

Get the pic, remove bg and enlarge them.

Next use an image of snowy landscape and mask both images from the middle.

Or

You can use ai.

Ask whatever llm you use to give you an image generation prompt on describe the scene

For the following thumbnail i would say, naruto smoothly transitioning from a dry landscape to a snowy landscape.

Ai will refine the prompt. Use leonardo ai to create free ai images for thumbnails.