If you're using Photoshop, then just render up some clouds, do 2 difference clouds, run a lighting effects (use a spotlight and play with the sliders until happy) then just crop out a mountain you're happy with. Of course Terragen 1 and Bryce 5 (both free) can make more photo-realistic mountains but I'm not proficient with them to any great degree, I just try to make PS do what all of those other things do as well as I can.