For every one or two designers willing to do it without receiving money, there will be a few more who send you hate mails for not offering money. They get quite angry and will say "beer won't pay my bills" and things like that but you may get lucky and land a talented student willing to do it for free in exchange for beer. In your ad, it might be easier to request a college, high school student or newbie to the industry sort of thing. The prima-donna type freelancers looking for real work will let you have it. They do in my area anyway.