The chance to divert an attack to a Mirror Image is skill -30 as a %. So at 120 skill it's 90%.
To make the most out of this you are better off using it on a character without Parry or DCI, as only connecting hits can be diverted, and when an attack is diverted to an Image the monster loses target and has to reflag on you. Providing there are no damage over time effects in play, or hit spell, this can usually give you well over 5 seconds of time without any incoming danger at all, while you continue to hit. Against monsters that counter attack your attacks triggering counter attacks will cause the reflag to happen quicker.