Pixabay offers searchable, high quality images you can use for any purpose without citation. It is much more comprehensive than Unsplash and it is very easy to use. I highly recommend it any time you and your students are creating presentations.
The only caution is that Shutterstock sponsors the first row of pictures, so you have to train yourself and your students to avoid clicking on those images. While they are sometimes the best looking images on the page, they are never free.