http://stackoverflow.com/questions/2068344/how-do-i-get-a-youtube-video-thumbnail-from-the-youtube-api
For the standard definition version of the thumbnail, use a url similar to this:
http://img.youtube.com/vi/<insert-youtube-video-id-here>/sddefault.jpg
ex: http://img.youtube.com/vi/JZd7XH6-_Es/sddefault.jpg
video ID: JZd7XH6-_Es
YOUTUBE GET SMALL THUMBNAILS
http://i1.ytimg.com/vi/j0YOXVlPUu4/default.jpg
where j0YOXVlPUu4 is the id of the youtube video
on custom fields:
<img src="http://i1.ytimg.com/vi/<?php the_field('youtube_video_url') ; ?>/default.jpg">