Entering or winning the contest does in no way transfer any rights to your work or give me the rights to use the work in my book. If the work of the winner gives me confidence he/she could make the illustration the first of a series of 10..15 illustrations, I'll make what I believe is an appropriate and fair offer for that series given the quality of the first illustration, but the prize of the contest will in no way be part of price that I'm willing to pay for the series. These are completely seperate.
Chances are, as an earlier contest with a short story showed me, none of the contestants will give me enough confidence in their abilities to produce a whole series of illustrations for my book, so indeed the contest, for me would likely end up being just for fun. This means the prize for the contest must be a just for fun prize.
Hope I'm making sense.