Just because you find an image doesn't necessarily mean you can use an image. Under U.S. copyright law, any image maker automatically gets the rights to his or her own image. That means the owners of copyright have the exclusive right to reproduce, perform, and distribute their work. Given these rights it might seem a little scary to use an image in a report, a lecture, or in an online presentation. However, the copyright code does offer some exceptions to these rules, known as Fair Use. The idea behind fair use was to carve out areas where one could use copyrighted materials without permission because the use would be for the greater cultural good. This way, journalists, teachers, critics, and artists would have the opportunity to use these materials in a way that doesn't interfere with the copyright owner's rights.
Fair use isn't black and white. There are no uses of an item that would be guaranteed under the law as "fair." The Fair Use Clause gives us four factors by which we can determine whether or not a use is fair. These are:
Deciding whether the use of a work is fair or not is a lot like using a scale. Your use may not be weighted on the side of fair use for each of the four factors, but it might meet enough of the factors to be considered fair. For example, when an art history teacher shows an image in class, he or she must how the entire, creative work. However, the character of the use is educational and the use doesn't negatively impact the market for the work, so this use is generally thought to be fair.
If you decide you can't use an item under the Fair Use Clause, you should seek permission from the owner.
Always give credit or cite the image! In scholarly settings, you will want to cite the image, and in commercial settings, you will want to use a credit line.