That does no good. You can present any image to a camera you want. You can't solve that problem by requiring another camera also being fed an arbitrary image.
The problem is that you want to ensure that an image corresponds to some real object. This problem is unsolvable, because a camera does not sense "real objects". It senses photons, and we already have the technology to shine any arbitrary combination of photons that a camera sensor can detect onto a camera sensor. There is no possible "signature" to apply to the output of a camera sensor that can validate that it is the result of "real objects", a definition that would get fuzzy if you tried to really nail it down in the presence of hostile attackers deliberately gaming your definition anyhow.
Besides, the entire idea that a photo is a concrete, specific thing is a very Hacker News, computer-programmer idea anyhow. Photos aren't just files. Long before we had the tech to shine arbitrary photons onto CCD camera sensors, photos were a process, not a single file. Even if we stipulate that the image on film can be reasonably called a "file", itself a rather large stipulation, the final photo would be the result of substantial decisions in how the image was developed. The program "Photoshop" is called that precisely because "photoshops" had numerous tools to affect the image at that point in the process, many of the tools in Photoshop are still named after these processes. Photo competitions absolutely include these elements; if there have ever been any photo competitions that accepted film as the input and rigidly developed all contestant's films the exact same way, they are the vanishing minority. Photos have always encompassed post-processing as part of their identity. A signature on a file is no good. You'd have to fundamentally rewrite the entire photo stack to include all the transforms, in a completely official and 100% specified manner, all signed, so the final signed photo has the complete record of everything done to it from the source... it's theoretically possible but absolutely not going to happen, especially in light of the fact the source is still meaningless for the above reasons. The whole idea is comprehensively unworkable.
The problem is that you want to ensure that an image corresponds to some real object. This problem is unsolvable, because a camera does not sense "real objects". It senses photons, and we already have the technology to shine any arbitrary combination of photons that a camera sensor can detect onto a camera sensor. There is no possible "signature" to apply to the output of a camera sensor that can validate that it is the result of "real objects", a definition that would get fuzzy if you tried to really nail it down in the presence of hostile attackers deliberately gaming your definition anyhow.
Besides, the entire idea that a photo is a concrete, specific thing is a very Hacker News, computer-programmer idea anyhow. Photos aren't just files. Long before we had the tech to shine arbitrary photons onto CCD camera sensors, photos were a process, not a single file. Even if we stipulate that the image on film can be reasonably called a "file", itself a rather large stipulation, the final photo would be the result of substantial decisions in how the image was developed. The program "Photoshop" is called that precisely because "photoshops" had numerous tools to affect the image at that point in the process, many of the tools in Photoshop are still named after these processes. Photo competitions absolutely include these elements; if there have ever been any photo competitions that accepted film as the input and rigidly developed all contestant's films the exact same way, they are the vanishing minority. Photos have always encompassed post-processing as part of their identity. A signature on a file is no good. You'd have to fundamentally rewrite the entire photo stack to include all the transforms, in a completely official and 100% specified manner, all signed, so the final signed photo has the complete record of everything done to it from the source... it's theoretically possible but absolutely not going to happen, especially in light of the fact the source is still meaningless for the above reasons. The whole idea is comprehensively unworkable.