Set a custom featured image for a gallery
By default Sunshine will use the first image uploaded to a gallery as it’s featured image. However, you have the option to choose a custom featured image.
When adding/editing a gallery, in the right column find the Featured Image box.
Select any image you want as the Featured Image.
If you want to use an image that is not part of the gallery, first upload the image to your Media area and then it will be available when setting the Featured Image.