I have seen, in my almost 30 years of collecting and 25 as a dealer, literally thousands of signed DiMaggio photos and other items. The variability is extreme, and so is the range in value. There are just too many factors to consider, and I have zero information on the piece you have. So any number I throw out will have virtually no meaning.
As for the authenticity, it depends on who authenticated it. PSA/DNA is good. Joe's Collectible Shack, not so good.
I've seen DiMaggio signed photos authenticated by PSA/DNA sell for $100 and I've seen others sell in the thousands. A mass-produced 8 x 10 reprinted photo that Joe signed at a card show or some other event is going to be at the low end. A Type 1 original photograph signed by Joe around the time the photo was taken and first printed, from his earliest days with the Yankess or even when he was in the Pacific Coast League, would be toward the high end. And the bigger the photo, the more valuable. Anything 16 X 20 or bigger from that early time period is extremely rare. The vintage photography market has been very active the last few years and values on just original photos have risen substantially. Throw in a vintage signature, and it could be well into the thousands.
But there is also a ton of common, modern stuff out there. Joe signed quite a bit until his final years. His will always be a desired signature, but it's not a scarce one.