Royalty Free Resources
It's important for teachers to model and teach digital ethics. These are a few of my favorite methods for finding royalty free pictures and videos. Check out RCPS's Tech Toolkit for an even larger collection of sites offering Royalty Free images, videos, and sounds.
If using Google with students, teach them to filter Google image content by usage rights. GAFE tools will automatically filter for this when searching for images within Slides/Docs/etc. On an iPad, you'll need to use the Creative Commons site to search Google images.
Learn to filter Youtube content for videos marked as creative commons to reuse and remix into your projects.