How to Select Which Attachment Serves at the Thumbnail of an Asset
Admin and Collaborators have the ability to reorder the attachments on an asset to change how viewers will see them. This also allows you to adjust the display image that is shown as the thumbnail.
1. Click on an asset and then click on "Edit Image" (or hover and choose the Pencil icon).
2. Click on the 'Advanced' tab and upload a new image in the section for 'Custom Asset Thumbnail.
PRO-TIP: You must first save any new attachments just uploaded to the asset before reordering.