How to change YouTube preview picture
YouTube allows you a choice of three preview images that show before an embedded video is played.
Start the process of changing the picture by logging into your YouTube channel. Select Upload > Video Manager. Click on the edit dropdown box and select info and settings. That will show you three different pictures from your video. Select one of them and click save change.
That will produce your new preview image. However, it may take up to 36 hours before your new image will display.