On this video Aaron Nace from Phlearn demonstrates how to create a really cool cracked skin effect using Adobe Photoshop.
On the first part of this two part tutorial Nace shows how to combine the two images (the original portrait image and the cracked texture image). An important part of this process is wrapping the texture over the face. Another part is the use of the liquify tool (including some interesting tricks with the freeze mask). One of the last things done on the first part of the tutorial is using the multiple blending layer to combine the two images.
On part two Nace demonstrates how to enhance the cracked face features and add blur to make the image look even more “realistic”.
Part two of the creating the cracked skin effect in Photoshop