I realise that you have a tight deadline; however, here’s my critique…I hope that it may help you as you plan your next project. 
In my opinion, your scaling of elements in this illustration is part of the reason that the overall composition isn’t satisfactory. The other issue is your use of value. When I am looking at your illustration, I don’t feel that you are trying to lead my eyes in any particular way around the illustration. I don’t have one particular place to focus on, no place I need to look at first and then an area where I should finish. And there’s so much going on! In any case, check out and compare the following:
- lady’s hand
- chains
- lady’s feet
- lady’s left knee
Look how large you’ve made the lady’s hand so as to appear as though it’s very close to the viewer, yet the chains that are attached to the manacle at her wrist are exactly the same size further down when they are supposed to be below her feet…the feet that you’ve made significantly smaller to appear more distant to the viewer.
That left knee is still causing problems. It appears as though it is above her hip because of its size and how you have it lit.
The value of the lady’s dress is the same throughout. Considering the effort you made with the scaling of the figure, plus with a major light source in the magic applied behind her, I would have expected the dress to be darker as it comes closer to the viewer. You need to pay more attention to how you use value to convey depth and areas of interest in your illustrations.
The feet are much improved! Excellent references.
I know that there is a CGTAlk artist (one of the moderators, Lunatique maybe?) with a thread offering paintovers of artwork somewhere on CGTalk…if he’s still doing that, I’d suggest taking this illustration over there and requesting one. You may learn some other points to consider for your next project to help you continue improving as a professional artist. Also, I’d highly suggest posting over on ConceptArt.org in the “It’s Finally Finished” forum and ask for some hard critiquing, if you haven’t already (don’t recall seeing your stuff there). There are some serious professional illustrators and concept artists over there, some of whom do quite a few book covers as well, in both traditional and digital mediums. Hopefully, they’ll be able to give you some good pointers, too.
Other than that, keep practicing!!
t
P.S.
As it is, my own artwork is so loose and unfinished these days because for the most part I only do rapid concept illustration for product development (pencil and digital) at work, that I rarely post anything over here or there…and have been doing alot of refresher studies of my own on the weekends so that I can start working on and posting finished illustrations once again. I’ll be sure to let you know when I’ve got a new project going so you can help me figure out where I need improvement. 
