How to know when a photo has been manipulated

So your friend showed you a photo of him with this hot chick. You don’t want to believe it but it looked like the real thing. So how do you become an image forensic expert to see if the photo is manipulated or not?

You don’t have to be. Just go to Image Error Level Analyser to upload the JPEG image and let the computer detect if any manipulation has been done to the photo or not. It’s pretty geeky how the owner of the site explains how the algorithm works:

“Error level analysis (ELA) works by intentionally resaving the image at a known error rate, such as 95%, and then computing the difference between the images. If there is virtually no change, then the cell has reached its local minima for error at that quality level. However, if there is a large amount of change, then the pixels are not at their local minima and are effectively original.”

-Neal Krawetz, Ph.D. http://www.hackerfactor.com

Yeah don’t try to understand it but just see how it works.