The most used types of thumbnails are the custom ones that include your business name, brand
name or logo.
This will help your video stand out and look professional.
Another option is to include representative images from the video so the viewer has an idea of
what it is about.
Finally, one popular choice is to create a thumbnail with your image and the title of the video.
This makes it personal and people generally like this.
Don't forget to keep this consistency for your future thumbnails as well.