Galleries covers should be enticing and encourage your visitors to enter a gallery. Other times, you want them to be more practical. For this reason you can easily customize which image is used for your gallery covers.
In the My Images & Videos panel, simply hover over an image thumbnail to reveal the Set as cover image button. Keep in mind that you can also customize thumbnails by editing the display styles. This goes for gallery covers, too!
You can also set cover images from images in other galleries. For example, if you want to create a custom thumbnail without having the image in that gallery, you can create a hidden gallery and upload your custom cover images. Then, use the cover image button and set the cover image for your other galleries.