Find a background to use. Flat surfaces that aren't facing directly toward the camera work best. A crack or two in the surface is even better.
Duplicate the image layer, and desaturate it. Now applay a Guasian Blur of 1 pixel. Save this as "shadow.psd"
Remove visibility of the grey layer and add a new layer. Draw or cut-and-paste the image you want to carve. Make it black and simple. Colors won't show up well later, and the more complex the design the harder it will be for people to tell what it is.
Use Free Transform to fit your design into the perspective of the background.
Set the layer visibility to "Overlay"
Apply the Displacement filter to your design layer at 5% Horizontal and 5% Vertical. Select "Shadow.psd" as your displacement map. This will distort the design to the contours of the background (incidentally, this is also how I got my reflection to match the water in the UFO over Toronto picture). If your design is farther away, decrease the percentages (although less than 3% is barely noticeable). Or, if your image is up closer or you want a more "rugged" look to the edges, increase the percentages. Also, if the image is tilted horizontally more than vertically, you can raise the Horizontal percentage and lower the Vertical to get more of a perspective.
Raise the Brightness by 100% and lower the Contrast by 100%. The design should all but disappear into the background. (alternatively, you can raise the Luminence until the design vanishes).
Now hit your design with an Outer Bevel with these settings: Chisel Soft, Depth 100%, Down, Highlight Mode: Overlay (75%), Shadow Mode: Overlay (75%). Then add an Inner Shadow with "Blend Mode" set to "Multiply." The other settings, such as Opacity and Distance are variable depending on your picture. You can change them to add more depth to the carving, match the surrounding shadows, etc. Also, if your image is close-up, or you want a "hard edge" feel to it, use Chisel Hard instead of Soft.
And that's it! Make sure that if you have something in the foreground, like a climber, that you move them to an additional layer so they show up. You can also use them to make the carvings more "interactive," like I have here with the guy's hand in Kokopelli's butt.