Many images are subject to copyright too. When doing a Google Images search, refine your search results based on different types of usage rights.
A work in the public domain is not protected by intellectual property law. The public owns these works, not an individual author or artist. You can freely use a public domain work/image without obtaining permission.
Selected public domain image websites:
More readings about using public domain images/work:
Creative Commons is a non-profit organization that "provides free, easy-to-use copyright licenses to make a simple and standardized way to give the public permission to share and use" an author's creative work with a Creative Commons license chosen by the author. Read the license of each image to decide.
Selected Creative Commons platforms:
More readings about Creative Commons and its licenses: