Okay, this is off the top of my head, as I'm out of town and don't have Photoshop right now. It sounds like you already have your head cut out, so I'm not going to go into detail on that.
1) Once you have your head cut, and placed on it's own layer over the mountain go to image->adjustments->black and white. Accept the default settings.
2) get a nice stone texture that you think matches the texture of Mt. Rushmore. (Here's something a quick google search turned up:
LINK)
3) copy and paste the texture over the head, then right click the texture layer and select "Create Clipping Mask"
4) set the texture layer's blending mode to either overlay, or multiply. You'll want to try them both to see which one looks better. This should give your picture the appearance of a stony texture.
5) Adjust the opacity of your texture to taste. Duplicating the layer will make the texture more distinct. (If you need your texture to be more distinct, without the texture's color effecting the image too badly, make the texture black & white like we did to the head)
6) Create a new layer above the texture layer, then hide all the layers below your head. All you want to have visible is the now textured head.
7) Make sure you are working on the new, top layer, and go to image->apply image. The new layer now has a flattened version of your face and texture on it, so go ahead and delete the original face and texture layers. You should also make your invisible layers visible again at this point.
8) select the eyedropper tool (hotkey 'I') and select a color off of the original Rushmore stone that you think is characteristic of the original stone.
9) on your head's layer, get out the magic wand tool (tolerance of 0) and click anywhere in the empty space around your head.
10) go to selection->invert selection. You should now have the entirety of your head selected.
11) on a new layer, above your head layer select the brush tool and fill in the selection with the rock color that you grabbed with the eyedropper. Set this layer's blending mode to "Color"
Alright, I
think that will do it. Like I said though, that's off the top of my head so it may not be exactly what you're looking for. If you have any questions, feel free to ask. Also, if someone else around here has a better idea, please share it with us.
