The thing is, you're asking for work which you have no intention to pay for (unless you happen to like it.)  This is assuming artists are not like regular workers, in which artists should or should not be paid based on the employer's discretion.  I wouldn't recommend any designer fall prey to such a method, unless you state clearly that you must pay at least one artist in this contest for their work, and that all artwork created will remain under the copyright of the artist or at their discretion.

I recommend, instead of holding a contest, holding an auction, where you start the bidding at, say, 1 BTC, and then designers bid lower and lower based on how much they'd like the job.  No guarantee you'll get any takers, but, I don't think you should expect professional work for .1 BTC.  Logos take time to create, and work without the promise of pay really isn't worth the trouble for any professional.