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 )
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
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