120,000 cover images loaded into Picards memory, let's say 100kb per image (low-balling), that's 12 gigs right there (early morning here, but I think I did that right...)
edit: nope it's too early, it'll load one image per album I suppose, but then again you'll probably be getting some images that are 2mb+...
You'll find it easier to work with smaller batches anyway, Picard (and any other tagging program) needs some manual checking to work precisely
This raises a more serious issue/question though -
Are you running Picard on a backup/ sample of your files first?
Picards basic functions are to retag, rename and move files (depending on your settings). It will overwrite existing information by default. Please be very very careful before hitting save on 120k files.