Before you copy/paste an image (photo, graph, cartoon, etc.) from Google, first make sure you have permission to do so! Look for images with a Creative Commons license by using the Search Tools button on Google Image Search.
Run your search using subject keywords. From the results page, click on the Search Tools button. Then select the Usage Rights drop-down list for the appropriate option.
Another great source for images with Creative Commons licensing is Flickr.com.