If you've uploaded a video and you find that the image you uploaded alongside the video is missing, then you will need to add a new image to the video.

To do this click on Menu -> Your channel name;

Then click on 'Videos';

Click on the video that you need to add the thumbnail image to then choose 'Settings';

At the right hand side of the screen just above 'Video State' you should see a little upload icon, please click on this to add a new thumbnail image.