Public Domain or Creative Commons media are images, musics, video, etc., which, although have a copyright, are available for public use. All media still needs to be cited, with the individual who created it being named as well as the website it was originally found on. For example:

"Image from - John Doe: www.anywebsite.com" 

Below are some copyright friendly sites that contain images that are available as creative commons material for you to use in your projects.