To change the thumbnail or cover slide for your wideo, play the wideo until you get to the image you want as your thumbnail. Pause it, then click the camera button in the top left corner of the player. You may have to wait a minute and/or refresh the page, but it will change shortly!
This will be the first image to be shown to viewers before they press the play button in your videos. You may want to pick the best part of your wideo to encourage or seduce viewers to watch it.
To change a thumbnail for your wideo on YouTube, please consult YouTube.com