Images found on the internet represent intellectual property and are generally not free to be copied or used by others.
When using images in a research assignment they should be treated as a source of information. The source should be included in your bibliography and when you insert the image into your assignment you should provide a reference to the author/creator and year of production (intext citation).
There are some sites; however, which supply free images for use by the public. See the links below to a few of these sites.