1. Navigate to the Library > Gallery Categories area.
2. Click on the settings drop down box (Cog icon) of the gallery category you would like to edit and select Edit.
3. If you previously hadn't set a cover image, then you will be able to use the Set cover image button in centre of the grey thumbnail.
If there is already a cover image, you can use the Change button below the thumbnail.
4. Select a gallery.
NOTE: Only galleries that are in the gallery category will appear in the drop box.
5. Finally click on an image to make it the new cover image.