Summarize the following:
A good way to figure out what kind of layout you want for your own thumbnail image is to look at thumbnail images used by different YouTubers. You can add transparent clipart and pictures to your image to make it more eye-catching. Make sure your design is relevant to your video, since misleading thumbnails violate YouTube's Terms of Service. Since this is what gets associated with your video title, make sure the design compliments your video's title and also gives an idea of the content in your video. Make sure not to repeat the title of the video in your thumbnail, since it'll already be displayed clearly next to the thumbnail once it's been uploaded to YouTube. Since thumbnail frames are rectangular, it'd be better to design your image so that it fills the thumbnail frame (square images leave blank space).

Summary:
Look at thumbnails for YouTube videos you watch. Customize your design. Format your thumbnail around the title. Make a rectangular image.