Open your image in Photoshop and duplicate it so that you have two identical images. Select your top image and choose "desaturate" from the menu. Decrease the opacity of the desaturated image to about 65 percent. You should have somewhat of a degraded image. Play with the opacity until it has the appearance that you desire.
Open your skull image and drag it on top of the face of your image. Choose the reshaping tool to fit the skull image onto the face of your image. Once the skull fits nicely on top of the face of your image, select the layer mask, which will allow you to blend the skin into the skull image. Select the brush tool and choose medium as the hardness. Use the brush to blend the skull into the image.
Select the cuts-and-cracks brush and lightly go around the face, adding these effects to give your image a greater zombie feel. Once you are satisfied, load the veins brush and add veins to the image. You should now have a creepy zombie image. Convert the layers into a single image by selecting "save as JPEG image" from the menu.