Center
Max Width 400px
Wide
Max Width 800px
Full
Max Width 100%
YouTube Image Sizes
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.
Player Background Thumbnail (480×360):
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 (120×90)
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 (120×90)
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 (320×180)
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 (480×360)
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 (640×480)
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
Source taken from here