After a Visual Discovery Service is created, the first crawl run and it has been viewed in the 3D environment, a thumbnail is generated as a "preview" of what's in your environment. Whenever a new version is created and opened, a new thumbnail will be generated.
This thumbnail can be updated in the 3D environment. Head over to Settings in the top-right corner and click "Update Thumbnail".
Now you will be able to capture a new image, you may zoom in and out until you find the perfect frame. This will be the new thumbnail of the Visual Discovery Service.
A new thumbnail was created: