Images that are provided for free online may only be used for the purposes that are expressly permitted in their license agreement or in ways that pass the fair use test. Images released under a Creative Commons license may be used and shared, with some restrictions.
The fair use test allows people to re-publish images that were created by others for reasons such as academic work or the reporting of facts. Copying an image created by a third party for the express purpose of making a profit or marketing a product is not considered fair use. Some Creative Commons licenses allow work to be used, modified or shared commercially.