This fantastic website comes thanks to Jenny Baccon as a great way to find images that already have the proper citation built right into them. Head on over to http://www.photosforclass.com/ to find a ton of great images that are ‘safe’ to use (as well as adhering to our goal of being Principled users of media).
Once you have searched for your image, hit the ‘Download’ button and grab the image. 
The only hitch is that the images tend to be quite high quality, so I draw your attention to the post from a couple months back: How to Re-Size Images Using Preview.