Open access and public domain images are images that you can use in your own projects for free.
One way to search for open access images is through Google Images. After searching for your keywords, on the results page, click on the "Tools" button beneath the search bar. From the menu bar that appears under it, select "Creative Commons licenses" from the "Usage rights" drop-down box.

Many museum websites also offer open access and public domain images. Refer to the Art & Architecture tab for a list of museums with extensive open access image collections.