Photos 8 is a great place to find thousands of images that are in the public domain. These images can be used in any way that you and your students see fit. There are twenty-two categories of images of which the largest collections are of animals, birds, and sunsets.
I learned about Photos 8 through a great blog post on Making Teachers Nerdy. I encourage you to read that blog post for more public domain image resources.
Applications for Education
Other than using images of your own creation, using images in the public domain is best way to create a digital presentation. Using images in the public domain means that you don't have to worry about what is or isn't fair use or do you have to Creative Commons attributions.
Here are some related resources that may be of interest to you:
Creative Commons Animal Photos
Video Introduction to Understanding Fair Use
Copyright for Educators