Look at it from the designers perspective... Instead of paying a single designer to do the job, you are asking a bunch of designers to do free work, THEN if you like it, pay for it (in this case, with a prize pack which probably has the value of a fraction of what the work is actually worth).
I don't steal your music, then if I decide I like it, I buy it. I listen to the freely available single you release and decide to buy from there. You should look at an artists freely available portfolio, and decide to hire from there.