When you create a post and add a video file to it, the Catholic Social Media system will automatically recognize it and give you an "Add cover photo" option on the bottom lefthand side of the video preview:
Once you upload a cover photo, that will serve as the main image for the post inside of Catholic Social Media and as the cover image for that video on all social media networks.
If you do not include your own cover photo, the Catholic Social Media system will automatically choose the first frame of your video (which may be white, black, or an unflattering image). For best results for parishes and followers on social media, be sure to include an eye-catching cover image.