Whether or not it's plagiarism depends on who actually made the image you're using. Google image search conveniently lets you filter by license type. If you click "Search Tools" right below the input bar in your search, it will open a second line of options, among which is "Usage Rights."

Most publications by the US Government are in the public domain and can be used without limit and without credit by anyone at any time. A handy source for the kind of information you want is the National Atlas home page.
Quote Originally Posted by http://www.nationalatlas.gov/help/faq.html#tweleve
Nearly all information collected by the Federal Government is in the public domain and use of National Atlas Data produced under this project is not restricted in any way.