If I've got a trigger that says "a unit is attacked" and a condition specifying which unit was attacked in order to kick the trigger into actions, would a mirror image of the unit still set off the trigger?
Depends, they count as "Unit Type of X" but not as "Unit is equal to Variable" either way, adding the condition "Event Unit is an Illusion equal to False" makes it so illusions will never trigger the event, no matter what.