Human to Zombie
Open the image you want to modify. Here is mine.
Now, open the cracked earth image, copy and paste it into the photo you want to modify. To copy and paste, open the file, go to EDIT->SELECT ALL. then go back to your project image and go to EDIT->PASTE.
If the cracked earth image is not big enough or too big, you can resize it so it covers the area you want to apply this effect.
Now just change the Layer Mode of the cracked earth layer to "Linear Light", and change the opacity down to about 60%.
Now simply use the eraser tool and erase out any area you don't want the cracks to cover, in my case, I want her hands to appear normal, only her face is cracked. You may need to occasionally change the size of your brush so you can erase smaller or larger areas.
So that's it, you can do alot more from here, just play around with it and see what else you can come up with. For example, I also erased out the eyes and lips part of the image and gave the original image a little sharpening "Filter->Sharpen->Sharphen"
Download my .psd file here.