Yes, there are a few libraries of public domain images online. None of them are all-inclusive though. The painting in my example came from WikiCommons: https://commons.wikimedia.org/wiki/Main_Page .
Knowing copyright law is hardly necessary, but here is Public Domain in a nutshell. When a picture, recording, or text gets published, this is typically the same moment that copyright protections begin. Copyrights do expire. The U.S. has the longest lasting copyright law in the world with a lifespan of 95 years. When a work leaves copyright protection for any reason (usually because the copyright expired), that work enters into an area of law called the Public Domain. Some people release work into the public domain immediately which is what NASA does with all of its astronomy photos. Some stuff gets released with a Create Commons license which is just as good as Public Domain status except that it sometimes comes with a requirement like you must credit the original artist when you use this image. Creative Commons is more often used for characters and books, but if you run a search on Deviant Art, you'll find plenty of CC and PD images. Archive.org is a great source for CC books and documents (as well as backups for dead websites, a very veryn handy resource). WikiCommons is a great source for photographs but also has a lot of old paintings.