Fair Use- Following the terms of usage listed for an image- aka. not ripping it off!
Here are some sites that make finding this out easier:
Creative Commons:Creative Commons is an organization, that set a standard for "fair" image usage and sharing. Find content you can share, use and remix. Do not assume that the results displayed in this search portal are under a CC license. You should always verify that the work is actually under a CC license by following the link.
Google Advanced Image Search: Can search by usage rights, image size, or within a site (like an institution or museum).
Flickr Advanced Search: Can search by creative commons licensing.
World Images:Can use images in educational setting if you credit the use.
Using images is a great addition to many presentations and papers. You must take care however, to adhere to the permitted use and terms of conditions in using these images. Many do not allow any use, and many require permission is granted by the creator first. You must make sure of the terms before you use the images in any work.
Many sites allow searching by use. Use the advanced search for sites like Flickr and Google Images. They have many images that allow for use by "attribution" in educational, non-online setting.