It'd be a real neat feature, though I wonder how much load it'd add to the server (I know nothing about this and how it'd affect things; don't take this as a question from a knowledgeable party). I reckon too many pictures wouldn't be a good idea, but a small space enough for past covers would definitely be nice... It's aesthetic more than functional, but I would not complain if it ever comes true LOL
It reminds me of the good old days when Warriors (cats) would have the map of the cat clan territories in the inconvenient start of the book, and it's like a solid 500 page doorstopper, so you never actually look at the map... Actually, having it as an addition to glossary might not be so bad. It can be a place to put maps or picture of cast...