Just like you would cite any textual source you use in a project or paper, you should give proper attribution for any images you include.  Each discipline has slightly different requirements, so check with your professor. 

The basic information you should always note includes:

• the title
• the artist/photographer/creator
• the source (i.e., information about the book, journal, database, or web address where you found the image, including page numbers and publication information).

Check the appropriate style manual to find out what to do for each citation style.