These sources will help you find images that are freely available for you to use (so long as you cite them or provide information on their creators). Here is more information on how Creative Commons licensing works, and how to identify items in the Public Domain:
Here is the general MLA format to use when citing online images. The rule is to fill out as many of the citation elements shown in the example (while understanding that some of them are difficult or impossible to identify):
Creator’s Last name, First name. “Title of the digital image.” Title of the website, First name Last name of any contributors, Version (if applicable), Number (if applicable), Publisher, Publication date, URL.
Here is the same sort of format to use in APA for citing an online image:
Artist or Author. (Year of image creation). Description or title of image [Image format]. Retrieved from http://www.someaddress.com/full/url/