PhotoShop will be montrously difficult to make units with, as it's a 2D graphics ap. What you need is a 3D ap. I know some folks have been successful with Poser 4, but in my assessment, Poser is too limited. Anyway, I don't actually know as I've never tried.
In addition to the 3D application, you also need some mad 3D skills to be successful. It's a very complex process, and making good units (see ReAnimators work in Hexographica, or Mormith's beautiful re-modeled Forest Giant in my own map, Acts of Piety (http://creation.thinktivity.com/pages/piety.html), Cydonian's work in a bunch of maps, or Santa's work in the WW2 series) takes a huge investment of time and energy. If you decide to pursue it, good luck!
