Before using any image you find on the web, make sure that the owner does not object.
At the very least, be sure to correctly cite where you got the image.
Images or pictures that you decide to use in a presentation or research papers must be cited. Only those that are obtained from royalty free clip art, such as the clip art available in Microsoft Word or Powerpoint do not need citing. Any items obtained from the web or scanned from a print source should be attributed to the owner of the copyrighted work.
As a general rule, the following elements are needed in the citation:
For citing video, art, sound, and more, see instructions at the Yale Writing Center.