If you are using copyrighted images for your presentation, it can't hurt to put in a disclaimer statement like: "This educational [presentation, poster, slideshow, etc.] contains images protected by copyright law. When possible, images were used as permitted through Creative Commons licenses or available in the public domain."
Here are some sites to use when looking for images:
Last name, First name Middle initial. Title of photograph. Original date of photograph. Title of Online
Collection. Date of posting. Current location of original document. Database. Sponsoring organization. Day Month Year of access <electronic address>.
O'Sullivan, Timothy H. Incidents of the War. c1865. Selected Civil War Photographs from the Library of
Congress, 1861-1865. 12 Jan. 2000. American Memory. Lib. of Congress. 2 Aug. 2005
Below are examples of how you can briefly credit an image source in your school work: