Your use of any images should be bsed on whether the site itself provides notice of someone claiming rights to the images, and the sites may disclaim copyright ownership with a notices like "This is a work of the U.S. Government and is not subject to copyright protection in the United States. Foreign copyrights may apply."
As a general rule, the U.S. goverment can't own copyrights, so depending on what kinds of images you mean - photos? drawings? -- they could be a work prepared by an officer or employee of the U.S. government as part of that person's official duties, and under section 105 of the Copyright Act (with some specific exceptions), they're not subject to U.S. copyright protection.
But -- the site could include the kind of creative works that are protectible by copyright, and you should assume that logos and trademarks of U.S. agencies are trademark protected like non-government trademarks, which can be used for nominative fair use purposes to identify the agencies with proper attribution.
One thing to look out for is a government web site is developed or maintained by a contractor, where parts of the site authored by the contractoris copyright protected, in accordance with the terms of the website contract. It's also possible that copyrighted works owned by others may be posted to government web sites. Copyrighted works that aren't owned by the government should be included on government web sites only with permission of the copyright owner and should include an appropriate copyright notice so you'll know who's claiming the rights.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
