If you retrieved the image from somewhere like Britannica Image Quest or Flickr, they usually come with a Creative Commons license allowing reuse for educational purposes.
Images for decorative use - cover page
If the image is placed on the cover page of the document (or similar 'decorative' use), you would only need to credit it. Examples of how to display this credit can be found in Screenshot 1 (taken from Britannica Image Quest), but see the Creative Commons wiki linked below for how to create the various elements.
Screenshot 1
Images used to illustrate a point - within the text
If the image is used to illustrate a point in the body of the text, you would need to reference the image according to a given referencing style.
See other links below for more information on image copyright.