In a recent patch, mirror images now grant experience when killed (also mirror images do 15% of the Blademaster’s damage). For most ways you can kill a mirror image (attack it, purge it, devour magic on it, nuke it with spells like nova or storm bolt) you get experience, as expected. Killing it with towers does not grant experience, as expected (since killing things with buildings never grants experience).
The weird case is when using hex on a mirror image which instantly kills the illusion but doesn’t grant experience. I don’t know if not granting experience is an intentional decision given that even a level 1 hex will still instantly kill a level 10 mirror image, but it’s unintuitive and to my knowledge the only case where using a spell to kill an enemy doesn’t give XP, so I’m raising a bug report in case this isn’t intentional.