Whether you are using them in a paper, Power Point presentation, Prezi, or video, you must always provide a citation for your images, just as you would any other source. Image citations are usually provided in captions, which can be reproduced in a list of illustrations or an appendix to your paper. Sometimes, images are also cited in bibliographies and notes.
The format of your citation will vary depending on the citation style that you have chosen to use, but it will most likely include the following information:
Artist's / Creator's name
Title of the work
Date the work was created
Current location of the work (museum or other repository)
Place that the work was created
Dimensions of the work
Material / Medium (oil on canvas, woodcut, etc)
Information about the source that you acquired the image from
Most citation style manuals will contain information about how to properly cite images. For examples of image citations in MLA, Chicago, and APA format, take a look at the following resources:
Individuals are responsible for not only finding the images to use in their published works, but also for securing copyright permission to use them. Here is Bucknell University's guide to copyright information, along with other copyright information.