This short and easy tutorial teaches a very neat photo manipulation technique you can use to make a part of human body look like it's decomposing. I suppose you already know how to work with layers and blending modes, if not read the tutorial about making blends.
First, you need two images: one of a human body part, and another of a cracked tree bark. Here are mine:
Open a new document, and copy the body part on the base layer. Copy the bark image to the layer above and set it's blending mode to Color Burn. Duplicate the bark layer by pressing Ctrl+J and set the copy's Fill to about 25%.
Now take the eraser tool and select the bottom bark layer (by clicking on its icon). Decide which cracks you want to leave, and erase all others from that layer - the more careful you are while erasing, the more realistic it will look. Since other layer is partly visible, the skin won't be as clean as a healthy skin would be, which is offcourse what we want :)
You should get something similar to this. As you can see, I deleted cracks covering the hair and eyes on both layers, and the large cracks from the face only on the 100% visible layer. I did this in 2 minutes, so it's not even close to perfect, but you get how it's done. You can improve this a bit by taking a step further. There are now some gaps with no cracks, and let's say you want them to be cracked as well. That's when you choose the Clone Stamp tool . Hold Alt and click on the part of image you want to be copied. Now (without pressing Alt) paint over the empty part where you want to copy it. It takes a few shots to get it right.
Okay, this sure looks gory and repulsive. Excellent :) Now merge all layers and play with Color Balance (Ctrl+B) and Hue/Saturation (Ctrl+U) a bit to get the colors right - we don't want this zombie to look too alive, so the colors should be a bit colder. Here's my result. Aww do I look adorable or what? ;P
You don't have to stop here, though - tree images are a helpful resource of pseudo veins, cracks and gore - just what we need in dark manipulations :) Have fun!