When you add a video to the galley, the default placeholder image will display before playing a video. You can override this with a nice custom thumbnail that gives a finishing touch to the video gallery.
The setting can be found under Gallery > Choose a Video > Custom Thumbnail
Turn the Custom Thumbnail option ON and set an Image.
You can adjust the Overlay Color. See How to Set Overlay Color on the Video Thumbnail on Mouse Hover?
How to Set a Custom Placeholder Image for the Video?
How to Set a Custom Placeholder Image for the Video?