[實用] Youtube Thumbnail

(Image size depends on video.)

• Player Background Thumbnail (480x360 pixels)
  http://img.youtube.com/vi/<insert-youtube-video-id-here>/0.jpg
  
• Start Thumbnail (120x90 pixels)
  http://img.youtube.com/vi/<insert-youtube-video-id-here>/1.jpg
  
• Middle Thumbnail (120x90 pixels)
  http://img.youtube.com/vi/<insert-youtube-video-id-here>/2.jpg
  
• End Thumbnail (120x90 pixels)
  http://i1.ytimg.com/vi/<insert-youtube-video-id-here>/3.jpg
  
• High Quality Thumbnail (480x360 pixels)
  http://i1.ytimg.com/vi/<insert-youtube-video-id-here>/hqdefault.jpg
  
• Medium Quality Thumbnail (320x180 pixels)
  http://i1.ytimg.com/vi/<insert-youtube-video-id-here>/mqdefault.jpg
  
• Normal Quality Thumbnail (120x90 pixels)
  http://i1.ytimg.com/vi/<insert-youtube-video-id-here>/default.jpg

And additionally, the next two thumbnails may or may not exist. For HQ videos they exist.

• Standard Definition Thumbnail (640x480 pixels)
  http://i1.ytimg.com/vi/<insert-youtube-video-id-here>/sddefault.jpg
  
• Maximum Resolution Thumbnail (1920x1080 pixels)
  http://i1.ytimg.com/vi/<insert-youtube-video-id-here>/maxresdefault.jpg

• 1st Thumbnail Image, Small (120×90)

  http://img.youtube.com/vi/bQVoAWSP7k4/1.jpg
  

  3rd Thumbnail Image, Small (120×90)

  http://img.youtube.com/vi/bQVoAWSP7k4/3.jpg
  

  2nd | Default Thumbnail Image, Small (120×90)

  http://img.youtube.com/vi/bQVoAWSP7k4/2.jpg
  

  Default Thumbnail Image, Full-Size (480×360)

  http://img.youtube.com/vi/bQVoAWSP7k4/0.jpg