If you've ever used the 'video' content block in the Zephyr Editor, you may have noticed that some videos preview with some black bars in the preview image. This is actually being caused by the size of the thumbnail attached to the video. If you're not using your own video here, there may not be much that can be done about this of course, but if it is your own video, you may prefer to update that thumbnail with one that is properly sized to eliminate those black bars -- keep reading to see how that needs to be done.
The ideal size for this thumbnail is a 16:9 aspect ratio, exactly 1280x720 pixels to be specific. If you can craft an image that is exactly 1280x720 px to use as your thumbnail, you'll be all set.
How to update the thumbnail:
Since the thumbnail itself needs to be updated in youtube, we recommend following these steps here provided from Google: https://support.google.com/youtube/answer/72431?hl=en
https://youthumb.net/ - this tool allows you a quick look at what thumbnails youtube is delivering to platforms like rezora when that thumbnail preview is used. If available, the 'MAXRES' version will be used, or if not available, falling back to the 'STANDARD' version instead. If you see black bars here, your thumbnail may need some work.