The Folder Chooser in a Gallery shows once a client enters their Gallery past the Gallery cover. This is where they can choose which folder they'd like to view.

You can specify the photo that displays as the cover for that folder. By default, the first photo in a folder is chosen as the folder cover photo. To change the folder cover photo, right-click any photo in a folder while editing the gallery and click the "Set as Folder Cover" button.

Alternatively, you can select the image and click the "Set as Folder Cover" button in the top bar in the "Photos" section.

After uploading it, you'll be prompted to crop the image. Choose the most important focal point of the image from the copping tool to ensure that part is always seen. Click "Save" to save the image.