Ok, for those people that seemed to have missed it, I'll explain the joke.
The calendar that Nick picked up says "Twelve Month of Cottontail - Playbunny". So, the photo of the rabbit from the back with it's tail up and legs spread is supposed to be flashing vag.
I couldn't quite understand the picture of the fox at first. I thought he had a potbelly but I think that's supposed to be his prepuce (the skin covering the penis in most mammals) and thus well hung for a fox.
I might be wrong with my assumption but I believe that's the joke the artist was going for.