According to the Library of Congress, "copyright refers to the author's (creators of all sorts such as writers, photographers, artists, film producers, composers, and programmers) exclusive right to reproduce, prepare derivative works, distribute copies, and publicly perform and display their works. These rights may be transferred or assigned in whole or in part in writing by the author. Unless otherwise agreed in writing, work created by an employee is usually owned by the employer. The U.S. Copyright Act gets its authority from Article 1, Section 8, cl. 8 of the U.S. Constitution."
The following are USC databases and digital collections that contain copyrighted and reusable images.
According to the circular, "International Copyright Relations of the United States" from the United States Copyright Office:
When using images from other countries, you will need to be aware of international copyright laws and ask the author directly for permission to use their image. For use within the United States, the following guidelines apply:
Works published outside the United States before July 1, 1909 are considered to be in the public domain. [Fishman chapter 18.12 and 18.15]
Works published outside the U.S. with a U.S. Copyright notice before 1923 are considered to be in the public domain. [Fishman, chapter 18.13 and 18.15]
Works made by the government of the United Kingdom (England, Scotland, Wales, Northern Ireland) and published more than 50 years ago are considered to be in the public domain. [Schultz, p. 219; Office of Public Sector Information web site - http://www.opsi.gov.uk/about/faqs.htm
Use Outside the United States
Through various international treaties, most nations have established reciprocity with regard to copyright protection. According to the U.S. Copyright Office, “Protection against unauthorized use in a particular country basically depends on the national laws of that country. However, most countries offer protection to foreign works under certain conditions that have been greatly simplified by international copyright treaties and conventions.” (Circular 38A [pdf], "International Copyright Relations of the United States," page 8.) If your use is going to take place outside the United States, you will need to be aware of the laws of the country in which you will be using the material and the treaties and conventions in which it participates. The list of U.S. copyright relations as of 2003 that is available in Copyright Circular 38A may be of assistance: Circular 38A [pdf].
Please be advised that this does not represent legal advice on obtaining permissions.
It is important to always cite an image used from another source. According to Purdue Owl's MLA Citation guide, provide the artist's or photographers' name, the title of the work of art or image italicized, the date of creation, the institution and city where the work is housed. Lastly, provide the name of the website in italics, the link to the website and the date of access. More information on citing images can be found below.
Example from Purdue Owl MLA Citation Guide:
Goya, Francisco. The Family of Charles IV. 1800. Museo Nacional del Prado, Madrid. Museo Nacional del Prado, www.museodelprado.es/en/the-collection/art-work/the-family-of-carlos-iv/f47898fc-aa1c-48f6-a779-71759e417e74. Accessed 22 May 2006.