Artist Kekai Kotaki is new to Magic, and did four cards in Zendikar. Art Director Jeremy Jarvis describes him as a "super-talented artist from digital gaming."
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 for Murasa Pyromancer:
Color: Red creature.
Location: Any outdoor.
Action: Show a badass, burly human shaman in cool traveling armor/cloak. He's armed with an arm-mounted, heavy-looking mystic fire staff, and has it hefted like a minigun (except it's attached to his arm -- maybe it and part of his arm are blackened from repeated use). He's in the act of blasting a gout of flame out of the end of it, toward an out-of-frame foe. It's so powerful that his body is set against the "kickback."
Focus: The fire shaman.
Mood: If all else fails, bring in the guy with the flamethrower staff.
The next step is for the artist to submit sketches of his or her vision of the art description.
Here's the initial sketch.
The creative team liked the direction, but asked that the gear look a little more weathered and less fancily designed, to keep the "shaman" feel.
And here's the card in its final state, as you can find in booster packs of Zendikar!