Note: YouTube is serving thumbnails from 2 servers:
//img.youtube.com//i.ytimg.com
Examples are with //i.ytimg.com server just because it’s shorter, no other particular reason. You can use both.
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Player Background Thumbnail (480x360):
WebP //i.ytimg.com/vi_webp/<YouTube_Video_ID_HERE>/0.webp JPG //i.ytimg.com/vi/<YouTube_Video_ID_HERE>/0.jpg -
Video frames thumbnails (120x90)
WebP: Start: //i.ytimg.com/vi_webp/<YouTube_Video_ID_HERE>/1.webp Middle: //i.ytimg.com/vi_webp/<YouTube_Video_ID_HERE>/2.webp End: //i.ytimg.com/vi_webp/<YouTube_Video_ID_HERE>/3.webp JPG: Start: //i.ytimg.com/vi/<YouTube_Video_ID_HERE>/1.jpg Middle: //i.ytimg.com/vi/<YouTube_Video_ID_HERE>/2.jpg End: //i.ytimg.com/vi/<YouTube_Video_ID_HERE>/3.jpg -
Lowest quality thumbnail (120x90)
WebP //i.ytimg.com/vi_webp/<YouTube_Video_ID_HERE>/default.webp JPG //i.ytimg.com/vi/<YouTube_Video_ID_HERE>/default.jpg -
Medium quality thumbnail (320x180)
WebP //i.ytimg.com/vi_webp/<YouTube_Video_ID_HERE>/mqdefault.webp JPG //i.ytimg.com/vi/<YouTube_Video_ID_HERE>/mqdefault.jpg -
High quality thumbnail (480x360)
WebP //i.ytimg.com/vi_webp/<YouTube_Video_ID_HERE>/hqdefault.webp JPG //i.ytimg.com/vi/<YouTube_Video_ID_HERE>/hqdefault.jpg -
Standard quality thumbnail (640x480)
WebP //i.ytimg.com/vi_webp/<YouTube_Video_ID_HERE>/sddefault.webp JPG //i.ytimg.com/vi/<YouTube_Video_ID_HERE>/sddefault.jpg -
Unscaled thumbnail resolution
WebP //i.ytimg.com/vi_webp/<YouTube_Video_ID_HERE>/maxresdefault.webp JPG //i.ytimg.com/vi/<YouTube_Video_ID_HERE>/maxresdefault.jpg