Video thumbnails let people see a quick snapshot of your video. After your video is finished uploading, you'll be able to choose a thumbnail from the default three options YouTube automatically generates. To select the one that you'd like to use, simply click the thumbnail and click save changes.
Here's how to change a thumbnail at any time −
If your account is verified and in good standing, you may have the ability to upload custom thumbnails for your video uploads.
To upload a custom thumbnail when you upload a video, follow the following steps.
Record or upload your video.
At the end of the video processing stage in the upload interface, you will see an option to choose a Custom thumbnail under the "Video thumbnails" section.
To upload a custom thumbnail for an existing video −
Your custom thumbnail image should be as large as possible, as the image will also be used as the preview image in the embedded player.
We recommend your custom thumbnails −