I agree with @doublethickcustard. As a professional illustrator, I've seen my fair share of "do this for free to me, and if I find it good enough compared to others who will have freely given their time and energy too, I'll reward you by giving you exposure in my project, for which I might get paid with real money and you'll get paid with many thanks and again, exposure !!! It's not as if I was asking you to work for free, it's just drawing, you like doing that, it's not work for you right ? So ! What do you say ?"
It's tiring and we have to change that kind of mentality, not encourage it.
I mean, if you've been in this forum for a bit, you've probably seen quite a few authors looking for artists to illustrate their work for free. Adding the word "contest" on your ad doesn't make it more legit, on the contrary since some people will give energy for no result whatsoever.
Don't get me wrong, giving exposure to fellow artists is a great initiative, I do it myself whenever someone sends me a fanart, I put it in my comic's gallery with a link to that person's profile or comic and I did a contest once but there were tangible rewards for the winners and though I put their entries (all of them, not only the winner's) in my gallery for everyone to see, I didn't use them in my comic.
When Tapas began their book section, I was hoping to see partnerships between authors and artists blossom so I really understand the initiative. But I STILL think the artists should get a small part of whatever profit you make out of this (now if you don't make any, forget everything I said).
And one last thing and my rant is over, I'll promise : you might wanna be careful with the way you phrase your announcement.
Here I read "I'm flattering you even though I haven't seen a single of your drawings so that you'll be more willing to take the challenge (talking to your ego here) I kindly "offer" you, aren't I nice ?"
"exposure exposure exposure even though you have no idea how much exposure I currently have with my book"
"I wanna make sure the drawings I get aren't crappy and the staff is behind me on this" The staff knows about this contest ? It doesn't feel like their way of doing things...
Anyway You want illustrations for your book. That's fair. Then you can say : "Hey, those of you who like my story, just know that if you wanna make fanarts, I'll make sure to include them in my story or in a special gallery" (who knows, maybe you'll meet a fan who'll want to create a real partnership with you).
Ah... but you want good quality drawings... That's fair too, I understand. But as soon as you add quality constraints and specs, then you have to pay the artists. Organizing a contest just to keep the ones you like is unfair.
Sorry for the rant, I'm sure you don't mean anything wrong with this (and maybe there are things I didn't get in your message) but I just wanted to give you an illustrator's point of view.