Yes you can - you can add pics to the actual file, then Album, Track, Performer, Genre, Director, ASIN, CDDB, Ref#, Channel, Episode, Program, Title, Disc ID, Conductor, Composer/Writer, Studio, Rating, Release Date, Website, Chapter ... in that order!!
Look in the Pluto_Media database in the AttributeType table for the PicPriority field that describes this. There is also a wiki article on some of the basics for picture selection but I can't remember where it is
My understanding is that if you do not assign a picture to the actual media file, it will then look for a picture assigned to the Album attribute, then Track attribute and so on down the list until it finds one. I guess you can even assign fall through default image by assigning it to one of the attributes you know will always be blank!