My opinion is that every job poster has different preferences as to how they would like to go about finding the right person to do the work for them. My concern with a one method fits all approach is that it will alienate job posters and they will just go elsewhere because they don't have to jump through so many hoops.
Some job posters don't wish to have their project posted openly for many different legitimate reasons, some don't care. Some project are simply too big and would require each potential to post multiple images which of course takes up the limited space we each have to directly upload images.
My two cents is to have different methods of posting work for hire.
1. Make available a list of qualified freelancers with links to either a small on site portfolio or larger off site ones. Let the poster be free to look through, and contact the freelancer(s) of their choosing. No having to go through all of the rigamarole, and person can be discreet if they so choose. Saves the site from having to micro-manage appropriate requests.
2. Contest style. Pretty much follows what we have been doing, but with the addition of aforementioned, approved, ideas.
Basically Job poster states description of what they want and any source material needed. Potentials would then complete
project(s) to the best of their ability and post in the original thread.
I like Hoogles idea of of an agreed upon end date when all the images would be approved at once to prevent idea stealing.
I also like Chads' idea of a standard site watermark being implemented.
This approach would give the people without a decent portfolio a chance to work on their skill and build one.
It also gives job posters the option to have multiple versions done of their project to pick from if they want to go that direction.
I'm not sure if a minimum price is necessary, I think the price sorts itself out. Meaning that if a job poster wants to offer $5.00 for a skilled, time consuming job, I sure as heck am not going to bite and many others wouldn't either. The OP is either not going to get a response at all, or he'll get one from an individual for whom either $5 is legitimate offer or from someone who wants the experience and doesn't care about the money. Either way the OP is getting exactly what they paid for.
Well, I've rambled on for long enough, so I'll be quiet now. That's my two cents, spend it how you like.