You're probably seeing this because you're wondering how to add a thumbnail to your video.
If you are uploading your video to Facebook, Instagram or YouTube, it will let you add a thumbnail from there itself. You will not be able to do it with Rocketium specifically.
Uploading a thumbnail to Facebook- https://tubularinsights.com/change-facebook-video-thumbnail/
Uploading a thumbnail to Instagram- https://www.kapwing.com/resources/adding-a-custom-cover-to-an-instagram-video/
Uploading a thumbnail to YouTube- https://support.google.com/youtubecreatorstudio/answer/7024632?hl=en