Many students, and teachers, don't realize that you can't just use any image from the internet. You need to sight your source, and in some cases, you can't use the image at all.
To avoid copyright issues and Creative Commons License issues, use Public Domain photos.
Making Teachers Nerdy has a great post on this topic. It has a list of sites where you can find public domain photos and sounds. I encourage all teachers to read this article and share these sites with their students. They should also talk to their students about copyright issues. In K-12 the students may get away with copyright violations, but the consequences are very real and very severe once they get to college or a job.
Photos8 is a great site on her list that I have seen mentioned elsewhere and I have used. It has great quality photos and a large selection to choose from.