How to ensure the authenticity of a picture?

There are several solution for image authentication. The simplest way is by checking the EXIF. It doesn't validate the image as unedited, but if an EXIF tag left by Photoshop or other software is found, you can be sure the image is edited.

The law-accepted method is by using camera that sign the picture it took. Of course this won't detect optical manipulation, but at least it's good for ensuring forensic photo aren't tampered when the photo itself is taken by trusted officer. Also, when the private key is leaked, boom, every image is now suspect.

A more general method would be detecting the fluctuation in the image statistics. Software for such method can be extremely expensive for forensic situation or could be free. Then again, they won't actually detect an optical manipulation.