Glyph of Mirror Image

Glyph of Mirror Image is a minor glyph for mages. It buffs a mage's Mirror Image spell to show his or her mirror image copies casting Arcane Blast or Fireball instead of Frostbolt, depending on the mage's primary talent tree specialization.

Source
This glyph is created with Inscription (430). It is discovered through Book of Glyph Mastery.