Restrictions may exist for the images and videos shared in this guide. Most allow personal or educational use.
Creative Commons defines the different types of copyright agreements for their images. Flickr also provides copyright information - many images can be used as long as you give credit to the artist. Morguefile offers free stock photography that may be shared without attribution.
Ultimately it is your responsibility to understand the use guidelines for any image you use.
Royalty free or rights managed? Stock photo licensing online has a helpful FAQ. Basically...
Public domain? Sometimes it is difficult to assess whether a work is in the public domain, but when a creative work is considered in the public domain it is free to use as you wish. Creative Commons provides a detailed definition.