What is public domain?

The public domain consists of all the creative work to which no exclusive intellectual property rights apply. Those rights may have expired, been forfeited, expressly waived, or may be inapplicable.

When a work is in the public domain, it is free for use by anyone for any purpose without restriction under copyright law. Public domain is the purest form of open/free, since no one owns or controls the material in any way.

-Wikipedia

Images posted here are typically free to use but please check with the source institution for more specifics. Thanks!