Images used for a class assignment will usually meet the Fair Use exception for the purposes of copyright compliance. Cite all images used just as you cite textual information. [Fair use looks at purpose (educational or business?), the nature of the image (factual or creative), the amount of the source used (one image from an article?), and the effect of the use (will other users view your image instead of purchasing the article?). ]
If you do not meet fair use requirements, either get permission from the copyright holder or use images that are in the public domain or that have a creative commons license.
The following are general purpose image collections. Some of these sites will not provide the download size options and metadata that licensed sources do.