Depending on what kind of source you’re using, you’ll want to consider the different ways to credit the person who created the image, information or media you’re using.
The first step should always be whether you have the rights to use it. Is it available on a site like Creative Commons with a link to a license giving you the right to reuse it? You’re good to go. Did you find the picture on a Google search and pull it from the results? You can’t use it before you’ve checked with the photographer or news organization where it originated. What about that person’s Instagram post or Facebook video? Even though it’s out there for the world to see, you didn’t create it, so you still have to ask for permission to use it.
(Read more about this in The Media Handbook chapter on attribution.)
Here are some sources you can use to find images, video and audio available for reuse:
Flickr Creative Commons
Google advanced image search — be sure you’ve selected the option for photos free to use or share
UW–Madison campus photo library — you can only use this for UW-related student projects, and you must credit the photographers