You can find images online that are labeled for reuse, but conditions for reuse vary widely. Always check the use terms for any images you find. Public domain images are those without any use restrictions and are copyright free. Anything that isn't labeled as public domain will have some kind of license attached.
Keep in mind that the principle of fair use applies to using images as well to other material.
Try some of these sites to find scientific images. Be sure to examine the terms of use for specific images you find.
Anatomy Image Bank from the U. of Minnesota -- Has copyright-free images as long as they are used for educational purposes.
Openstax Science Resources and Textbooks -- Check the CC license details, but most are CC-BY licenses.
CDC Pubic Health Image Library -- Most of the images are public domain, but some are copyrighted and have use restrictions; check terms for each image you find.
NIH (National Institutes of Health) Scientific Images -- NIH's Flickr site. Check license terms for each image. NIH has other image albums here.
Anatomy of the Human Body -- This is Gray's Anatomy, 1918 edition at Bartleby.com. The 1918 ed. of Gray's Anatomy is in the public domain. Also see the illustrations from Gray's Anatomy on Wikimedia Commons.
PEIR Digital Library -- Pathology Education Informational Resource (PEIR) has images for educational use.
PubMed and Openi -- After searching, your search results list present images in the right column; click on the link to view all images. Each image will have conditions for reuse.
Clinical Anatomy -- Illustrations presented under CC BY-NC-SA 4.0 license.
Berkshire Community College Bioscience Image Library -- Quality microscopic and macroscopic images with CC 0 license.
Smithsonian Open Access -- Thousands of 2D and 3D images available for reuse from the Smithsonian collections.
General Image Search Sites
Wikimedia Commons -- Has both public domain content and images with open content licenses (see explanation here).
Pixaby - High quality images, both photographs and illustrations. Images are free to reuse, but read the content license summary closely before reusing image.
Creative Commons Search -- Images, text, video and audio; can filter by CC license type.
Google Image Search -- Limit to images with Creative Commons licenses under the Tools menu:
