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.
Tineye is another excellent reverse image search.
"How to Identify Mysterious Images Online" by Tasha Bergson-Michelson gives step-by-step instructions.
Use this search to locate images that are licensed for sharing.
These search engines are designed specifically for image searching.