We have a requirement to delete authored image in Thumbnail tab of page properties via Authoring, in short we need to deselect image or remove authored image.
Just adding to @berliant 's note. Whenever your editable template includes a thumbnail, that thumbnail will be visible on every page that used that editable template. In this case, if you want to overwrite the page's thumbnail, you need to include an image asset in this path /content/my-site/en-gb/home/jcr:content/image.