Artist Steven Belledin has illustrated 68 Magic cards.
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 Steven was given for Putrefax:
Color: Green creature
Action: Show a large living mass of putrefaction that has arisen from Rey-Goor, the swamp at the edges of the Tangle. It is comprised of decaying matter (rotten gelfruit among other things), black sludge, tissue, and metal. Copper-scrap "teeth" protrude from its mouth. This is a green creature that has been tainted by Phyrexian influence.
Focus: The "Phyrexianized" elemental from the bayou.
Mood: A shambling, unstoppable abomination.
Armed with that art description, Steven submitted a sketch to Magic's creative team:
Looks great, right? Right! Let's see the final version.
And there it is! The shambling abomination of your dreams!
And here's what the abomination in question looks like in Scars of Mirrodin packs: