Artist Steve Prescott is great! Coming in to Innistrad, he'd done around eighty pieces of Magic illustration.
The first step of the creation of a card's art is its art description. The art description tells the artist what the card will do, what its flavor is, and what the mood of the illustration should be. Here were the instructions Steve was given for the card:
Color: Green spell
Action: This is a symbolic piece that represents magically replicating creatures. Show a cameo-style portrait of the heads of two wolves. We see the profile of the two heads facing opposite directions in an oval frame. The space around the heads is filled with vines and leaves. Give some indication of the magic: the wolves' heads are surrounded by a halo of silver light.
Focus: The silver light around the wolves’ heads.
Mood: An orderly increase in the natural world.
Notes: Cameo refers to a portrait of two heads in profile facing opposite directions in an oval frame.
Steve, you can be designy/graphic with this if you like.
With that art description to work with (and the explicit encouragement to be "designy"), Steve submitted a sketch to the Magic creative team:
Parallel Lives | Sketch by Steve Prescott
Parallel Lives | Art by Steve Prescott
One advantage of seeing the art at this size is that hidden details can come out. The card has eyes!
Here's the final card as it will appear in Innistrad booster packs: