You add a thumbnail to your video when you publish it on social media. You can select an available image from the library or select one from your video. The selected image will be added to the publication and managed by each social network.
Internet users take less than 5 seconds to decide whether or not to watch a video. The image displayed before playback is decisive in making them want to watch it. If you don't, the hosting platform or social network will choose one by default from your video, which may not illustrate the subject or flatter the important person(s) appearing in it.
For a long time, most social networks refused to allow images (or thumbnails) not taken from your video, to avoid deception. Today, however, you can use any image you like. Just make sure that it enhances your channel and your video, and is not deceptive or misleading. Internet users will appreciate it.