The Collection Chooser in a Gallery shows once a client enters their Gallery past the Gallery cover. This is where they can choose which collection they'd like to view.
You can specify the photo that displays as the cover for that collection. By default, the first photo in a collection is chosen as the collection cover photo. To change the collection cover photo, right-click any photo in a collection while editing the gallery and click the "Set as Collection Cover" button.
Alternatively, you can select the image and click the "Set as Collection Cover" button in the top bar in the "Photos" section.
After it's been uploaded, you'll be prompted to crop the image. Choose the most important focal point of the image from the copping tool to make sure that part is always seen. Click "Save" to save the image.