The art, of course, is a reference to the Return to Ravnica card Tablet of the Guilds.
As a bonus, here's the art description that Ryan Barger was given for Guildscorn Ward:
Color: White spell (unguilded)
Action: Show a mage who is walking past the Tablet of the Guilds. A magical halo glows around him. We also see that as he has passed the guild symbols on the Tablet, they have crumbled and flaked off in response to the mage's passing.
Focus: The crumbling guild tablet.
Mood: The guilds have no power over me.