screenshot verification

I do not know if this is possible but what I want is to have a program that can validate a screenshot? For example, we all know screenshots can be faked and manipulated. Is is possible to create a script or program that would look for the signatures in a file of a jpeg or similar graphic file and validate the file has not been tampered with by a photoshop / MS paint or other editor program? Any method can be used. I ultimately want a program that will verify a screenshot pic has been modified or edited from its original state. I don't know how to get any more descriptive than that. All programmers bidding must understand what I want here. And can create either a desktop program or a script that can be run online to perform this task. I dont know how to set a budget for a program like this. I do not know if this is a simple task or a complex one. Please bid accordingly and PM me why you can do it or if you can do it. The job will be awarded to the programmer who has a reasonable bid and timeframe.