It uses the first image that matches the rules in jukebox.ini in the folder the song lives in, so if you have multiple albums in one folder the juke will get confused.
In jukebox.ini:
# CoverArt defines the order in which files are searched
# for cover art. Cover art is only used for CD skins
# at this time. The cover art found in the flder for
# the first song in an album is used.
CoverArt1 = folder.jpg
CoverArt2 = AlbumArt_{?


?-?

-?

-?

-?



??}_Large.jpg
CoverArt3 = *.jpg