When hosting videos through GoAnimate, the ability to select a specific thumbnail is controlled through the GoAnimate video maker. Many subscribers also host videos on YouTube. When hosting on YouTube, the thumbnail a viewer sees before viewing a video is controlled by YouTube (not GoAnimate).
If you don't like the thumbnail selected by YouTube, you can change it in a couple of ways. This article explains one of them (refer to YouTube for assistance with other methods).
When uploading a video to YouTube, select the Custom thumbnail option, as shown below, and upload an image. This image will be used as the thumbnail.