[douglas]

Depends on how obvious the origin of the spell you cast is. A Lightning Bolt coming from your fingertips is a dead giveaway as to where you are, of course, but a Summon Monster just making something appear with no apparent link to the caster would probably fool most monsters. For a Fireball, I'd require a spot check for the monsters to notice where the bead came from, and a bonus to the DC to notice it didn't come from the image if it originated near or even passed through the image. Of course this is all just my take on things, Jack's the DM.