You always want to make sure you have the appropriate permissions whenever you reuse an image that you did not created. Images (and other works) are automatically protected under copyright the moment it's done. Even images that are not marked with a copyright can be protected by copyright. In order to avoid any questions about whether you can use images, look for images that have an open license or are in the public domain.
Normally, even when you use this type of image, you'll need to do a couple of things:
Note: Most of this content was shared by Amy Hofer from Open Oregon Educational Resources. Please check OOER for the most recent version.