Use these resources to locate images that are licensed for sharing.
Use the Google Images search feature to find a matching or similar image.
Drag your mystery image into any Google search bar that contains the camera icon, or click on the camera icon and paste in a url.
"How to Identify Mysterious Images Online" by Tasha Bergson-Michelson gives step-by-step instructions.
Tin Eye is another reverse image look-up. It may have indexed a page that Google has not.
These search engines are designed specifically for image searching.