Thanks to all suggestions so far, although as Charley Noble said this could be one of those songs that is all the more interesting just because it doesn't make sense!!

If Snuffy is right then the transformation of "as fair as an image" into "as fair as any midge" would have to rank as one of the best mondegreens of all time, and the ghost of old Zozimus must be roaring with laughter!!!

As Kevin rightly says there isn't any sense in looking for a "definitive" version here, as the variations help keep this song alive, but it is intriguing to find out what other people think is going on in the song.