If a photographer X takes a photograph of me, then X is the copyright holder. But since I as the person who was photographed have certain rights, X having the copyright is not enough for X being allowed to publish it.
Now X puts up the photograph on your website, and I don't like it. The DMCA doesn't actually give me any legal way to complain about this. I would have to declare under penalty of perjury that I am or that I represent the copyright holder. I do neither.
Since the DMCA doesn't give me any legal way to send you a takedown notice, you can't act on one and the DMCA can't give you any legal protection.
On the other hand, if I sent you a nice letter explaining the situation and asked you politely to remove the picture, and if you removed the picture, then I would have got the same result that the DMCA wants to achieve for copyright holders. I would be much less likely to take you to court. And if I took the photographer to court and included you in the court case, a judge would likely recognise that your prompt action reduced any damages and hold that in your favour to some degree.