U.S. Geological Survey Photographic Library - All photographs within this collection are public domain; therefore, signed releases, permissions, and payment for use or reproduction are not required.
AGI's Earth Science World Image Bank - for personal or educational, non-commercial use.
NOAA/National Geophysical Data Center - Natural Hazards Image Database - Usage: credit both the source (person / organization) and NOAA/National Geophysical Data Center (Details)
The British Geological Survey has an incredible repository of images.
NASA Visible Earth - A catalogue of NASA images and animations of our home planet (freely available to use with attribution - see Terms of Use)
Also see:
Other geology images free to use for non-commercial, educational purposes: