Why should a freelance artist absorb the risk for YOUR concept? Whether or not your idea is going to be successful is YOUR gamble, not ours. If you want talented people to develop good ideas (and increase your chances to be in the 1-10% of profitable content) you should pay your creators. Even if the contract is “work for hire” (meaning the creators do not retain any of copyrights for the characters they create), they need to be compensated for their time and the expertise which they are providing. You can find fair rates in the Graphic Artists Guild Handbook - Pricing and Ethical Guidelines.
—cartoonist and illustrator Alec Longstreth, from his public reply to a request for spec work. I highly recommend you read the entire thing, whether you’re a professional artist — it can’t hurt to remind yourself that your work is valuable and you deserve fair compensation because you are a professional — or a potential client, who might not even realize that spec work is not as normal as you think, and that everyone does not share your enthusiasm for the Bright Bonanza you’re sure is awaiting you in Hollywood.