Thumbnails are the image that appears in the player window before video playback begins. Using this endpoint, you can upload an image as a thumbnail.
Thumbnails are an excellent way to give people a visual preview of what the video is about.
For both a live stream or a video that is uploaded, the thumbnail file is validated to meet the following conditions:
The maximum size is 8MB
There is no minimum size
The MIME type can be one of the following: image/jpeg, image/png, image/webp
The thumbnail should be the same format as the video and use the same ratio for the best results. So if the video is in Full HD, the thumbnail would look best if it's 1920x1080 pixels or at least a 16:9 format.