Are you creating it yourself or rather searching for Stock Art? just very curious^^
Also something you could still do is contact Green Ronin. They bring the Critical Role specific Guide to Gencon and might be interested in your idea for such an add-on. Especially since the community is HUGE and would probably preorder or kickstart it if need be. And I bet you many of the Illustrations in the Setting Guide could be repurposed for Rule cards.
Or even cooler would be OotS (Order of the Stick for the uninitiated) style cards.