You can manually change the image that is shown as the the thumbnail of a folder if you like.
- Hover over the folder you wish to change the thumbnail of with your mouse to bring up Hover View.
- Click the ellipses to bring up the pop up menu.
- Click Set Image.
- Select the image from the folder you wish to be displayed as the thumbnail.
- Resize and position the image how you like.
- Click Save to update the thumbnail.