First, cool idea, being a designer I know how much a good critique can really help you polish your design and really improve it (so long as you don't take it personally).
Second, how does the process go once you do improve on your design and heed the advice given, can you upload your new design (replace the old one, show them side to side, represent a timeline of the iteration?) will it tell the people who critiqued that you followed their advice? and will your old design, having already been polished and finished, will it stay on there forever continuing to get critiqued on things that have been fixed already?
looks good I'll probably upload something soon, good luck
ps: if I did upload something are there any other incentives other than the critique (like maybe a linkback to a site) just curious.