Open textbooks can be defined as "textbooks that have been funded, published, and licensed to be freely used, adapted, and distributed." Typically open textbooks are available as a pdf, epub, or set of html pages that are available at no cost to students and instructors.
True open textbooks are licensed so that they can be freely printed, distributed, and even adapted with correct attribution.
To find OER, look for advanced search options in the tools that you use. In Google, you can find advanced search under "settings." Look for "usage rights" once you've reached advanced search.
Similarly, in Google Images, look under "tools" for "usage rights." These tools allow you to narrow your search to find materials that you can modify or reuse.