Sets - Setting the lead image
The thumbnail shown for each set in your gallery is called a lead image or set cover image.
For images in the same set
To assign a lead image to a set, navigate to the relevant set and open the Set information panel. Drag your chosen image over the thumbnail area. The page will refresh automatically to display your new lead image.
For images outside the set
You can also add a set cover which isn't in the same set. This can be useful for setting a lead image logo or graphic, without it becoming part of the gallery set itself.
To select the lead image from elsewhere in your account, navigate to the relevant set and open the Set information panel. Click on the thumbnail area, which will make visible a drop-down menu. Select your image from the drop-down and click the Update button.