You have to consider how the image (or other resource) is licensed. If the image is protected by copyright, you can only use it if it falls under fair use. Use one of the American Library Association tools to determine if the image is protected by copyright and whether your way of using the image is fair use.

If the image has a Creative Commons license, you must follow the terms of the license. Some owners will ask only that you include attribution. Others include stipulations on how and where you can use the image. Use these Best Practices for Attribution to guide the information you include in your citations.

The image below outlines all the possible Creative Commons licenses:
Creative Commons Polska
[Full size image]