Formerly tricycle/elm-embed-youtube. Search finds this package by either name.
Player Background Thumbnail (480x360):
PlayerBackground

Video frames thumbnails (120x90)
First | Second | Third

Lowest quality thumbnail (120x90)
LowQuality

Medium quality thumbnail (320x180)
MediumQuality

High quality thumbnail (480x360)
HighQuality

Standard quality thumbnail (640x480)
StandardQuality

Unscaled thumbnail resolution
MaximumQuality

Grab a thumbnail from your Youtube video