Lately in my company we have found a need to automatically check images, sent from client, whether are original or digitally modified in any way.
Research was done and some nice findings: most of the methods are only suitable for JPEG files – due to its compression nature.
There is this great paper that explains in details most techniques used in this particular field: http://www.hackerfactor.com/papers/bh-usa-07-krawetz-wp.pdf (also available here)
I have found couple of online tools that can also help you to discover that your friend played with Photoshop 😉
- http://www.imageforensic.org/ (backend: http://www.getghiro.org/)
And one offline tool: