Copyright is a form of protection provided by US law to the authors of “original works of authorship,” including literary, dramatic, musical, artistic and certain other intellectual works. The owner of the copyright has the legal power to use the work or allow others to use it for a finite period of time. The work loses this protection and is considered in the public domain if it was first created before January 1, 1923, or at least 95 years before January 1 of the current year, whichever is later; or the last surviving author died at least 70 years before January 1 of the current year.
Copyrighted, restricted clip art can not be used for commercial use unless you have a written agreement with the copyright holder.
Some copyrighted clip art is not open to any public use. For example, you'd be hard pressed to get a legal image of Disney's Mickey Mouse for display on your commercial website unless you are a licensed reseller of Disney products. Legally, you can't even copy an image of a Disney character to make your own personal T-shirt.