Copyright exists in a work at the time it becomes fixed in some tangible medium of expression. That means that an image you have downloaded from the Internet is almost certainly subject to copyright protection.
Copyright protects “original works of authorship”. Copyright does not protect ideas if they are only verbally expressed.
Exceptions include Public Domain and all materials created by the US Federal Government. The Public Domain should not be confused with information found in books, periodicals or on the Internet. The Public Domain is all those works that are no longer protected by copyright or never were.
In the United States, works before 1923 are usually in the Public Domain and are not subject to copyright. Nearly all works created after 1923 are subject to copyright protection. Copyright protection usually lasts for the author’s life plus an additional 70 years after the author’s death.