Google has updated their image search to include license type.  This means you can click the advanced tab on Google Image Search and narrow your selection of images to those that permit reuse.  If you’re just showing an image on the web to your class, this doesn’t really matter.  This is for instances when you need an image to publish with a paper, show at a conference, or produce any other work that will be publicly available.   You can check it out here:

http://images.google.com/advanced_image_search?hl=en

Keep in Flickr in mind as well.  The photo sharing site also allows you to search by license type.  The number of images are smaller than what you’ll find in Google Images, but the images themselves are usually of higher quality.