TimelineJS is an open-source tool that allows for the creation of interactive, media-rich timelines using a Google spreadsheet. Timelines allow you to use media, text, and dates to tell a story.
Some media is made available for others to use through special licenses. A popular license that does it the Creative Commons Attribution license, meaning it can be used freely as long as the creator is given proper attribution.
Search for works (images, music, & more) to use - CC Search
Google Images (can be filtered by Usage Rights under Tools)
YouTube (search results can be filtered by CC license)
Learn more about Creative Commons licenses - CC Licenses
Public Domain
Works that are not copyrighted are considered to be in the public domain. Some reasons they might be in the public domain are 1) they are very old, 2) their copyright has expired, 3) they were created by the federal government.
Use this tool to help you determine if the material you want to use is copyrighted - Digital Copyright Slider