I don't, as a rule, ever ask artists to contribute for free, but I still occasionally get gifted art from kind folks. (I'm more than happy to commission them for one-off work.)
Artists tragically undercharge for their labor, so I don't think the goal should be "coax them into contributing for $0" so much as "coax them into becoming an available and reliable talent pool for your community at an agreeable rate". If they're enthusiastic enough, some might do free work from time to time, but that shouldn't be the expectation.