If you choose to Upload a new file you can select a file from your computer or even connect to your Google Photos.
If you ever change your mind about the thumbnail image, just click on change or remove.
How do I set my video thumbnail?
On the right side of the Video Info View, you will find the Thumbnail tool. The thumbnail is the face of your video, so remember to choose an image that really conveys your video's message. When you add a thumbnail, you will be offered several options. You can upload a file, use a previously uploaded image or select one from alugha's marketplace.