How can I determine pictures that have been altered?

Question

I was just wonder if there was a program/site I could use use to determine edited pictures/screen-shots

Answer 1

I don't know of any program or site.  A lot of the time you can tell by obvious things, like the light is wrong, something is the wrong size, or you can see edges.  Even if someone is an expert, sometimes you can tell because the picture depicts something that's just impossible.  And sometimes you just can't tell.