To safely use images that you have found on the web without copyright infringement, you need to know the  difference between images that are in the public domain and those that are protected by copyright. Here are the  basics:

• All intellectual property (writing, images, illustrations, photographs, etc.) belongs to the person who created it, and that person may impose conditions or terms of usage on anyone else who wants to use the intellectual property. Failure to honor the terms of usage is a violation of the intellectual property owner’s rights.
• After a certain amount of time, intellectual property passes into the public domain. A work in the public domain is a creative work that is not protected by copyright and which may be freely used by anyone.

Anything  published before 1923 is now in the public domain. Unpublished works created before 1978 began to pass intothe public domain on December 31, 2002. The absence of a copyright notice on works published before March  1, 1989 puts them in the public domain (with some exceptions). Once an image has passed into the public  domain, there are no restrictions on its use. You can make changes, add color, resize, or even resell the image.

There is a misconception that images available on web sites are in the public domain. Usually they are not.  Images scanned from sources such as magazines, books, greeting cards, coloring books, etc. are copyrighted and should not be used without written permission. Most cartoon characters (Disney characters, for example) are copyrighted.