You can strike many adjacent foes with a single blow.
Benefit: As a standard action, you can make a single attack at your full base attack bonus against a foe within reach. If you hit, you deal damage normally and can make an additional attack (using your full base attack bonus) against a foe that is adjacent to the previous foe and also within reach. If you hit, you can continue to make attacks against foes adjacent to the previous foe, so long as they are within your reach. You cannot attack an individual foe more than once during this attack action. When you use this feat, you take a –2 penalty to your Armor Class until your next turn.
No. If your initial attack hit the caster, you can’t cleave to an image as if it were an actual creature. If your initial attack hit an image, you failed to hit your intended target (the caster), and therefore can’t cleave. As you can’t specifically target an image (because you can’t tell the images from the actual caster), you likewise can’t aim for an image and try to cleave to another image.
Can I take a 5-foot step in the middle of my attempt to use the Cleave feat, to bring another foe within reach?
No. Cleave is a special action and the conditions for that action are checked at the moment you begin your action. At that moment, all of the available targets are checked to make sure they adjacent to each other and within reach. You cannot take a 5-foot step in the middle of the action and check conditions again. If you do not have two targets within reach, adjacent to each other at the start of the attack, you could not even attempt to make an attack using Cleave.
Pathfinder RPG Core Rulebook. Copyright 2009, Paizo Publishing, LLC; Author: Jason Bulmahn, based on material by Jonathan Tweet, Monte Cook, and Skip Williams.