Always do the following:
Shiels. R. (2015). How to use Google Images Creative Commons [Video file]. Retrieved from https://youtu.be/8iTwL1SVCTY
Pixabay is a repository of Creative Commons images that are classed as Public Domain (where the images have had all rights cleared by the creators).
All images within this site do not require attribution or referencing, but it would be highly regarded in an educational setting if you included a statement somewhere within your work along the lines of:
For single images: "Pixabay (CC0)"
For all images within a work: "All images were obtained from Pixabay on a CC0 licence"
For a combination of CC0 and other images: "All non-attributed images were obtained from Pixabay on a CC0 licence"
This simplifies the process for if/when someone wants to reuse the image, they know where it has been sourced from.