From The Economist:

In an effort to reel in photography, camera-makers are making it more obvious when images have been altered.

One way of doing this is to use image-authentication systems to reveal if someone has tampered with a picture. These use computer programs to generate a code from the very data that comprise the image. As the picture is captured, the code is attached to it. When the image is viewed, software determines the code for the image and compares it with the attached code. If the image has been altered, the codes will not match, revealing the doctoring.

Another way favoured by manufacturers is to take a piece of data from the image and assign it a secret code. Once the image file is transferred to a computer, it is given the same code, which will change if it is edited. The codes will match if the image is authentic but will be inconsistent if tampering occurred.

Digital signatures, just as I suggested. But even that won’t be guaranteed:

…forgers have become adept at printing and rescanning images, thus creating a new original. In such cases, analysing how three-dimensional elements interact is key.

Yup. So we’ll also need the army of photographers, for independent views of the event in question, and an army of ever-more-sophisticated bloggers to keep the forgers honest (or at least catch them when they’re not).