The
taking of photographs is generally not where problems arise, unless you must violate a law in order to take the shot (ie. trespass). It is how you
use the photographs you take that will give rise to law suits (ie. infringement, invasion of privacy, defamation).
Taking a photo of a statue is often not a problem, even though the statue is an artist's creation and the creation itself is copyright protected. If the statue sits in a public place, you can take a photo of it. However, if you try to publish this photo of the statue, or use the image of the statue in a commercial manner, the artist may consider your photographic image a derivative of his protected work, and a suit against you could result.
The same would be the case with the graffiti artist (although if the graffiti artist was in danger of being arrested for vandalism if identified, he may not want to press the issue by coming forward with a lawsuit against a photographer who publishes his work
).
ANY case, however, depends on ALL of the facts and ALL of the particulars of a situation, so there is no definitive answer as to what would be copyright infringement until the infringement action reaches the court level and a court decides the matter.
With "commemorative" coins, the coins can be copies made (often in "gold") of coins already in existence (ie. a gold Susan B. Anthony dollar). A black and white photo of a gold-colored Anthony coin is not going to be infringement. However, if the coins are specially created with an
original design to commemorate, say, the passing of Charles Schulz, and the coins picture Snoopy and Linus or an original image of Schulz, then photographing and publishing images of these coins can be infringement (as well as a violation of trademark and publicity rights).
Again, it depends on the facts. Your questions are getting a bit into hypotheticals and what ifs, and there are not easy answers to these types of questions. A review of the Copyright Act can help, and case law can give you an idea of what decision a court could make based on similar facts and circumstances.
When in doubt, it is always wise to seek the advice of an IP attorney and/or get permission to use any potentially protected images directly from the creator of the image.