http://img.youtube.com/[VIDEO_ID]/[SIZE_PARAM].jpg
http://i.ytimg.com/[VIDEO_ID]/[SIZE_PARAM].jpg
Small thumbnail (120×90)
http://i.ytimg.com/vi/[VIDEO_ID]/1.jpg
http://i.ytimg.com/vi/[VIDEO_ID]/2.jpg
http://i.ytimg.com/vi/[VIDEO_ID]/3.jpg
Defaut size thumbnail (480×360) –
http://i.ytimg.com/vi/[VIDEO_ID]/0.jpg
Defaut
http://i.ytimg.com/vi/[VIDEO_ID]/default.jpg
Medium resolution
http://i.ytimg.com/vi/[VIDEO_ID]/mqdefault.jpg
Hight Resolution
http://img.youtube.com/vi/[VIDEO_ID]/hqdefault.jpg
Max resolution
http://i.ytimg.com/vi/[VIDEO_ID]/maxresdefault.jpg
Alternatively, you can use the YouTube API to get thumbnail images.
For more info about the subject
http://www.reelseo.com/youtube-thumbnail-image/
http://triotips.com/how-to/get-youtube-videos-imagethumbnail-url-768.html