Hover over the thumbnail with your cursor to expose the "+" icon. Click on it to upload custom thumbnail (upload an image from your computerfile system) or select a custom thumbnail (select from images that have already been uploaded to the collection in LUNA).
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Selecting upload custom thumbnail will open the Custom Thumbnail Selection tab. Once you have found the image you want to use as the custom thumbnail, click the "+" icon in the top right corner of the image.
