I advertised on Conceptart.org, at the unpaid job listings section, and I got a few replies, including a cool guy called Stephen Cousins. I asked all applicants to provide samples of their work/links to their galleries so I could take a look - I liked Stephen Cousins' work the best, and felt it best fit with the aesthetic I was going for.
Note that if you ask someone to do unpaid work for you, there does still need to be something in it for them. In some cases they may be willing to work just for the experience or to add something to their portfolio, but it helps if their work and name gets exposure (so they might be able to get paying clients in the future) or in some other way benefits them. Make it clear what exactly they can get out of it and make sure your estimations are realistic. Also keep in mind that if they work pro bono they're doing you a HUGE favour, so don't be pushy and keep timelines realistic (I advertised months in advance so that the person I picked would have time to put my work on the backburner whenever he had any paid work or other job prospect type stuff he needed to work on).
We worked together on the image, with me supplying as much concept material as I thought would be useful for each of the characters (screenshots, drawings, and a written description), and he sent me drafts and asked me for feedback/what I thought of them. We worked out the final design together.
He was very good to work with, but I think he deserves to get more paid work now.