The thumbnail selected displays as the portal preview on the Dashboard. To add or change the thumbnail for a portal, click the Options menu > Edit to access a portal.
Click Set Portal Thumbnail to add a preview or Update Portal Thumbnail to change the existing preview, then search for and select an asset to use. Thumbnails don't need to be assets that are already in the portal; they can be any asset you'd like. Only images can be used for thumbnails and previews.
If a thumbnail isn't selected, you'll see a default preview with the name of the type of the portal on the Dashboard.