When I need an image for illustrating a report with a generic example, I usually go to Flickr and look for images without copyright (or with a copyright that allows me to use it). You can actually look for the same at http://www.creativecommons.com
But perhaps the best option is to look directly for images tagged and classified by geologist! The British Geological Survey (BGS) has an awesome collection of images available online. This images are free to use for education, teaching, preparation and examination purposes, and surely will be welcome by many of you.