Each Component can have some additional metadata attached. Currently, the only metadata is a thumbnail. The metadata is edited in the right pane, Metadata tab.
The Component Thumbnail makes it easy for the marketeers to find the right component when creating an email. The purpose is to give a visual representation/thumbnail of how the component looks, which the user can rely on.
This is how it looks in the Email Editor, when thumbnails have been uploaded each Component:
In order to upload a thumbnail, take a screenshot/snapshot of the component in a state that is representative for the component. I.e. fill in some example data, to make it easy for the user to recognize.
Upload the image in the Metadata -> Thumbnail section.