Update Asset Thumbnail
The Upload Thumbnail page will allow you to upload thumbnails for selected assets. You can reach this page by selecting 'Update Thumbnail' in the 'Manage' Modal after an upload.
After dragging to the upload window, you should now see the image you want to apply as a thumbnail.
Click "APPLY" in the bottom right corner to confirm.
You will receive a brief popup confirmation image confirming that the thumbnail(s) have been updated.