Flickr: Encyclopedia of Life Images
Double check the rights, but most may be used for non-commercial purposes. Use the search box for the pool, lower left NOT the box in upper right.
Wikimedia Commons
Use the download function to get full resolution photos.
Pics4Learning.com
Free for educational purposes but not all images are high resolution.
US Forest Service Find-a-Photo
Click on "Search for Photos" or "Simple Search" and browse categories or enter keywords to find photos.