IAN Symbol Library. Integration & Application Network at University of Maryland Center for Environmental Science (IAN) has a library of symbols and images that may be helpful for building conceptual diagrams, and they are open source (meaning free for your non-commercial use): http://ian.umces.edu/symbols/#_Resources
Flickr. While not a bank for scientific images specifically, you might get lucky. From the home page type in your search terms, but then under the license drop down menu select the “All creative commons” filter. Creative commons is a copyright license that indicates the image is available for non-commercial use. https://www.flickr.com
ARKive. A source of free-for-use images of animals and plants—most are photos. http://www.arkive.org
NOAA Photo Library. http://www.photolib.noaa.gov
NOAA Fisheries Image Gallery. You might be able to find photos of species you work on here. http://www.nmfs.noaa.gov/gallery/images/
Paid Stock Photo Sites. Make sure you read the copyright rules. Some copyrights will cap the audience size that can view the image. For print products or unrecorded presentations, you will probably not have a problem, but for web-based resources, the number of viewers can be much less predictable over the long-term.