Is a Photo of a Photo Copyright?

Short answer: Yes. Less-short answer: pretty much what Zeroneg1 said. It's a derivative work, and you can't do that without the copyright holder's permission. Copyright covers the creation of something, not just the representation.

Is taking a picture of a picture copyright infringement?

Basically, copyright law says that when you take a photograph, you become the copyright owner of the image created. This means you hold exclusive rights to: Reproduce the photograph. Display the image in a public space.

Who owns the copyright of a photo of a photo?

Under U.S. law, copyright in a photograph is the property of the person who presses the shutter on the camera — not the person who owns the camera, and not even the person in the photo.