Why searching for anything? Just to get a ready-to-use code. Did you ever think about those who write the code? You can join. Why not writing is simply by the definition:
http://en.wikipedia.org/wiki/PSNR[
^]?
I'm not giving any advice on accessing the image only because you did not tag your Question properly, so I don't know what library you want to use. Is it WPF or
System.Drawing
?
It's look quite simple. Do it and ask a question if you face any problems.
—SA