All images on Pixabay.com are bound to the Creative Commons Deed CCO. This means that the person who associated a work with this deed has dedicated the work to the public domain by waiving all of his or her rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law. So what does this mean in plain English? Well, you can copy, modify, distribute and perform the work, even for commercial purposes, all without asking permission. The images are great, just avoid the first row of images which appear on a search as they have the Shutterstock watermark. Encourage your students to use this site instead of Google Images, teach them about Creative Commons and their responsibility as digital citizens.
Source: Richard Byrne (Free Technology for Teachers)