Always assume that an image, video, or audio file posted on the web can't be used without permission of the creator unless it has a license that states otherwise.
To find images available through Creative Commons and other licenses, see the resources below.
Curated collections of digital images and associated data for non-commercial, scholarly, non-profit, and educational use. Over 1 million images.
You can utilize search engines that allow filtering by license, including:
Be sure to check the specific license requirements of the file you want to use!