Images can be easily found online and in textbooks, journals, posters, etc, but not all images are freely available for you to use in your own work. Copyright, image licensing and other conditions can apply, affecting whether an image can be downloaded, scanned, reused or modified.
Take caution and investigate any copyright restrictions or terms and conditions of use before using an image. This may include photographs you have taken of people.
Creative Commons (CC) licences make content more useable, however you should always verify that the work is actually under a CC license by following the link. Since there is no registration to use a CC license, CC has no way to determine what has and hasn't been placed under the terms of a CC license. If you are in doubt you should contact the copyright holder directly, or try to contact the site where you found the content.