How to see the thumbnail from a youtube video

• Player Background Thumbnail (480x360 pixels)
  https://i1.ytimg.com/vi/G0wGs3useV8/0.jpg
• Start Thumbnail (120x90 pixels)
  https://i1.ytimg.com/vi/G0wGs3useV8/1.jpg
• Middle Thumbnail (120x90 pixels)
  https://i1.ytimg.com/vi/G0wGs3useV8/2.jpg
• End Thumbnail (120x90 pixels)
  https://i1.ytimg.com/vi/G0wGs3useV8/3.jpg
• High Quality Thumbnail (480x360 pixels)
  https://i1.ytimg.com/vi/G0wGs3useV8/hqdefault.jpg
• Medium Quality Thumbnail (320x180 pixels)
  https://i1.ytimg.com/vi/G0wGs3useV8/mqdefault.jpg
• Normal Quality Thumbnail (120x90 pixels)
  https://i1.ytimg.com/vi/G0wGs3useV8/default.jpg

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

• Standard Definition Thumbnail (640x480 pixels)
  https://i1.ytimg.com/vi/G0wGs3useV8/sddefault.jpg
• Maximum Resolution Thumbnail (1920x1080 pixels)
  https://i1.ytimg.com/vi/G0wGs3useV8/maxresdefault.jpg