Generally you'd use Silent Image because it's lower level and thus better for the job. But a very simple build would be Gnome Illusionist 10/Shadowcraft Mage 5/Earth Dreamer 5 with two flaws, taking Spell Focus: Illusion, Heighten Spell, Earth Sense, Earth Spell, Echoing Spell, Arcane Thesis: Silent Image, Residual Metamagic, and any other metamagic feats you think would be fun (Twin Spell would make sense). Get into SCM as fast as you can (Earth Spell gets you in earlier than normal) and take all the various gnome substitutions.
JaronK