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davidcyclebackI collect modern photographs. For a limited original photograph by a famous photographer, it is highly desirable and will raise the value if photo is signed by the photographer (In part because it shows that the photo was personally approved by its photographer). The difference is that the signature is almost never found or wanted on the image area. The signature will most often be found on the back, though often below the image on the front border. For the collector and photographer, the image is the thing of importance.
Even if they were willing to sign a baseball or magazine for you, many famous photographers would refuse to sign the image area of one of their original photos.
At more of a tangent, there was well known high end art dealer of original prints and paintings of folks like Picasso and Monet. Comparing the buying of a Picasso painting signed by Picasso and a poster reprint of the painting with the poster autographed by Picasso, she said the former was purchasing art while the latter was purchasing an autograph.