When you use an image in a paper or presentation, you must provide a citation. The exact citation format will depent on your bibliographic style (MLA, Chicago, Turabian, APA, etc.).
Most citations require the following information:
If you found the image in a book you will need the author, title, publisher information, date, and page, figure or plate number of the reproduction
If you found the image online you will need an access date, the web site address (URL) and in some cases an image ID number. Remember that an image displayed on a website does not necessarily belong to that website's author!
For examples of how to cite images in various styles, see the following guide from Dartmouth Library:
or this, from the University of Cincinnati:
RefWorks, a software that helps you manage your research and create formatted bibliographies, includes settings that allow you to easily produce references for images. RefWorks is avaiable FREE to UVA students. Click here for more information and to get started with RefWorks.