We all use images to illustrate the learning material. The advantage of Pexels is that you can browse a huge collection of images on different topics and use them freely.
You can start browsing by entering a term in the search box. Under the Explore menu, you can view popular collections as well as the best rated photographers.
It's also worth browsing the terms of use under Licence - what's allowed and what's not. In general, you can use and edit the images for free. Here you will also find some ideas on where to use stock photos and video.
After registering/logging in, you can create your own collection, so that if you need an image later, you can easily access it.
Some editing tools (such as the infographic creator Canva) are linked to Pexels, so you can access the collection directly within the application.
If you are a photographer, you can upload your own photos to the site.