I'll be honest here since I'm also an artist.
1- I rather know what the actual prize is, I wouldn't risk myself to "Maybe", unclear prizes or a cash price that doesn't compensate my time or effort, or its below my pricelist (As it wouldn't be fair either to my clients who pay what I demand).
2- I don't think this is for "All the artists out there" since in order to participate one must read the novel, that seems more like something a reader or fan would be more interested into. If this was for all artists out there, then I believe something like a short graphic description of the protagonist and maybe a few selected fragments that express their personality would be better for a contest, especially if its about being chosen as the official artist for it (Be it novel extras or comic adaptation).
3- This would look a bit better if it was a single novel, not for two. Try to make sure that can compensate what you demand to others. And even if it was for either fans or all artists out there, they still will interpret the characters on their own personal preferences based on the vibe and what they read, you could say at any time the art doesn't fit the story, but it'll still be free art you're getting. And I've witnessed way too many times in some contests, the judge "not liking" something but still taking it with them.
Is no accusation, but its a risk many artists are not willing to take.