Congress has deemed that murals are a "fine piece of art" so, just like an O'Keefe painting or a Warhol picture, a mural is protected against reproduction. If someone wishes to reproduce a protected piece of art, the artist must get paid, or give explicit permission otherwise. Without permission, the person copying the piece can be hit with a lawsuit.
When a mural is painted, it is immediately protected by copyright law. However, there is no legal proof that the artist did not take the idea from somewhere else, so simply painting the mural will not be enough to hold up in a court of law. Legally, the wall will be protected as soon as the correct copyright papers are filed and approved by the Library of Congress, where copyright records are kept.
Copyright law protects a mural against someone using the image and claiming it as his own. The law does not protect against someone taking a picture of the mural, unless the person uses the picture to turn a profit or create something else from the image of the mural, like a shirt.
The copyright of a picture will last for the entire lifetime of the artist, plus another 50 years. So, if an artist dies in 2010, the image will continue to be copyrighted and the estate of the artist will receive royalties until the year 2060.