Hi Guys , this is all About a crazy Zombie who trieed to chain saw his head :)
i am gonna use Photoshop CS5 here ,, which is of great use specially in merging a pic .
if u dont have adobe photoshop go to Adobe.com simple :) ( u might need to spend a lil more )
okkkk!!

This zombie trying to cut his head and wanna die and end up his undead life. he might have seen his gf's head in some dustbin

i always try all-most to start from scratch

Step 1: Getting Started : Open PhotoShop

So Just open Your photoshop CS5 or CS6 , or any photoshop ,tools to be used are same .

Goto File >> New >> height 780 pixels , Width 1024 pixels

i dont have chain saw so i downloaded one picture of person holding a chainsaw.
if u have a chainsaw come on hold it and ask ur mom to take ur picture n tell her ur gonna cut ur head !! she wont mind !!
(jokes apart : do not switch on ur chainsaw )

>>>one u got ur pic clicked just remove its background using Wand tool, marquee tool, or some other selection tool n press delete to erase pic

>>> i then downloaded a chainsaw which i feel was good for scene , i brought it in the scene to match the size and position .

NOTE : Make a duplicate of ur layer which u feel is gone wel n u can perform a mistake later on , also keep ur layers named .

here i duplicated the -man+old chainsaw- layer then i eased the white color from background. and Erased Old chainsaw

Step 2: !! Chainsaw Should Cut Ur Head Now !!

Now Once u have the chain say of same size and at same position let bend it a little which does not affect chainsaw just its blade to bent like modern chain saw now.

We are gonna use Puppet Wrap in edit menu or in image menu
in this tool u just feel like ur pic is like a chewing gum floating in air and u can fix some points in pic
ehen u have fixed some points just click and drag to stretch en the image

at end when u done make a series of back to back layer of chain saw and head because this will let u to have it in ur head ,
single layer wont do this , it will just look like its behind ur head , but duplication n bring it to front will solve purpose

after that just erase some parts of head so that it look like now the chainsaw is in head.

Step 3: !! Coloring Head !!

take a dark reddish color and use it on head front ChainSaw .
u can do it with simple brush tool.

>>>> Next use Smudge tool to open his mouth
in this i have downloded some brushes from websitehttp://brusheezy.com/ and search for a lot more brushes
i searched for cracks , Blood Spatter, Liquid flowing etc which may help to deal with brushes

example : http://www.brusheezy.com/brushes/38422-uh-oh-accident--splatter-brush-pack
Adding Brush is simple just drag and drop brushes.ABR on brush tool then u can see it in Brush library.

one u got a dark red crack on ur head
now u just need to darken its edges.
you can just use Blending properties with inner shadow
>>>just play around with inner shadow ,outer shadow, etc try using outer inner glows as shades of red

Step 4:

NOTE: the Exact values can be told but every one have pics of different expostures , scene which give different light effect on subject so one must try to add more reality to pic

Now all u have to do is add some ugly skin,Torn skin , Bones (not used),some bullet holes (not used ), some stabbing of weapon(used as AXE on back)

Step 5: !!! Axe-ing !!!

Bring Axe to the photo and same way bring as the CS
>place the axe behind the back erase back
>erase some parts of back
>do it red and then its done

Note : give at end a over all touch to pic so that it look scary and bloody :)