Generally, the following information, when available, should be included in your citation:
It is easy to find and download images from the Web. However, your use of these digital images downloaded from the Web or scanned from a print source may be restricted by copyright.
The Digital Image Rights Computator can help you figure out whether or not you can legally use an image.
Creative Commons Licenses: Creative Commons is a non-profit organization that has developed a standardized ways for creators to determine which rights they reserve and which ones they waive. Even though you do not need to obtain permission to use these images, they still require attribution (citation) when they are used.
Public Domain: Creative work to which no exclusive intellectual property rights apply. Those rights may have expired, been forfeited, expressly waived, or may be inapplicable. Because no one holds the exclusive rights, anyone can legally use or reference those works without permission.