As with many areas on the Internet, the copyright legalities are confusing and fall into the unexplored and are yet to be challenged in the courts. The concept of public domain, where the author has given up their intellectual rights and private copyrighted material is confusing to most of us. The safest thing to do is use one of the best paid clipart graphic libraries that gives you the unlimited right to use their graphics as part of the terms and conditions of the sale. That way you just never have to worry about it or accidentally use something not in the public domain on your website and get sued.