If you’ve created a collection, you can change the thumbnail by clicking on the pencil icon when hovering the mouse on the collection in the right pane. Select “Thumbnail” and choose if you want to upload your own photo or select one from your library. When you’ve selected an image, you can crop it, do some light editing of the image, add filters and add text. When you’re happy with how the thumbnail looks, click “Done” in the top right corner. Then remember to click “Save changes.”
If you want to create your own thumbnail the recommended size is 600x800px.