Hey @Deana Hudgins53! At this time, the social share image used for broadcasts will be the first image within the broadcast itself. If there are no images in the broadcast, then there won't be an image for the preview, unfortunately. (The thumbnail images for broadcasts are used for newsletter feeds59, not social sharing at this time.)
I can definitely see the value of being able to set your own social share image for broadcasts, though! I'd encourage you to file this as a feature request for our team here54 :)