Just like you would cite any textual source you use in a project or paper, you should give proper attribution for any images and/or videos you include. Each discipline has slightly different requirements, so be aware of what is required.
The basic information you should always note includes:
the title of the work
the date of its creation
the artist, photographer, or creator
the source of the work
the license type of the work
See Open Washington's Open Attribution Builder for works in the public domain or with a Creative Commons license.
A tool created by the Open Washington: Open Educational Resources Network
Facilitates the creation of attributions for images and other media types
Link to tool: http://www.openwa.org/attrib-builder/