When using any image from the online resources listed, make sure you click on the site's "Rights and Clearance" or "Terms and Conditions" section to make sure you are following proper usage policies.
When using images from the databases, sometimes you may use them for educational purposes [as long as you cite them properly and give credit (i.e. in a presentation)]; but, many images are still copyrighted, so always check their usage policies, too.