I've been working on a casino game as a way to learn XNA and I'd like to publish it to an open source host. The back end is coming along rather nicely, but I'm absolutely NOT artistically inclined, whatsoever --I know of no developers that are. I'm obviously not looking to make money off of this project, so offering to pay somebody for their time is simply not feasible. I'm sure there must be a community of artists/designers out there that would like to participate in an open source project, but where do I find them?
If the program works well, just publish it and wait to see what happens. Make sure people can easily replace your artwork, though.
You could try to launch a competition for the artwork, but I do not think too many people would submit anything if the project is new and unknown.
Check OpenClipart for some pre-made graphics, maybe you will find something suitable for your project.
Engage with designers through media sites such as Deviant Art and others. Designers are not as prolific as coders in open-source ... but we have one thing in common:
We like to showcase our skills
Just ask.
Having done quite a few Free Software projects myself, I'd have to say that you are lucky if you even get any extra volunteer coders. Even on a programming-related project you'd probably be doing good to have 1 full contributor and 3 patch submitters for every 500 users. Unless the project is art related, I'd imagine your rate of picking up volunteer artists would be even lower than code contributors.