Upload & Sell: Off
I'm running a design contest on 99Designs right now. The minimum is $299. I'm still in the first phase of the design process and so far it has been a mixed bag of entries. At first I thought that I was probably going to ask for a refund once the first round was finished but eventually I started to get some designs that I liked and opted to go ahead and guarantee the contest (meaning that a design would be chosen as the winner).
I used Illustrator to design my first logo and although it is "OK" it really isn't all that interesting. I wanted someone with a different perspective to take a crack at it and this has allowed me to see a pretty good variety of options. On the downside, the people who are competing are simultaneously working on other contests so you have to wonder whether they're truly giving it their best effort in terms of creativity. I think that in the final round you get to narrow it down to 6 designers and I would imagine that being a finalist might cause some people to step up their game. Having said that, I currently have a handful of designs that I'd be happy with right now so I'm eager to see what the final outcome will be.
If you do decide to use 99Designs I would suggest making it a Blind contest. I had several designers contact me and ask me to do that so others wouldn't copy their work.