Specifically, the blog post says:
This is a general call for the design of something that can go on the "backs" of our cards. Essentially, we hope to offer a pdf file that can be downloaded for people to print (after turning around the paper that has the card fronts). There are a few stipulations.
1. Best if the image is busy enough so that we do not have to print align card "backs" with card "fronts." In other words, we hope to have a design complex enough that can be cut in any way so that players cannot distinguish what they have in their hands.
2. Best if the image looks good in grayscale as well. For the option of printing in black and white.
3. Best if the image has some sort of biodiversity connection (this is the bit where you can fun with this).
Anyway, do go to this forum thread if you have ideas, suggestions, or submissions.