It's easy to find images on Google! When you run a Google search, you will find images listed in your search results. You can limit your search results to just images by selecting 'images' below the search box once you run a Google search:
For more precise image searching, go directly to Google Advanced Image Search. With Google Advanced Image Search, you can customize your image search by size, color, type (e.g. photo, animated, etc.), and usage rights. Usage rights allows you filter image results by license and find images that you are free to use.
There are a variety of online collections of public domain images. Here are a few you can try:
The British Library: Over one million images from a variety of 17th, 18th and 19th century books
New York Times Public Domain Archives: A collection of images published in the New York Times prior to 1923
Open ClipArt: A collection of thousands of public domain clip art images
Pixabay: over 470,000 free photos, vectors and art illustrations
PLiXs: Thousands of high quality images and video with CC0 license (aka: public domain)
Smithsonian Institution Public Domain Images: 3,400+ stunning images of people and places
For an extensive list of additional public domain image collections on the web, check out the Wikipedia: Public domain image resources.