Contain a variety of different types of images.
Most academic libraries, museums, and other cultural organizations have repositories of their collections. Some make materials freely available on-line. There are usually some materials that are only available by visiting an institution in person.
This list is just a sample. Do a Google search if you are interested in an institution not listed her.
When searching for images, keep track of as much of this information as possible to help you cite the image later in your papers, presentations, etc. Then you'll apply the appropriate citation style. See below for APA, Chicago, and MLA examples.
Be mindful of copyright infringement of images. See this infographic for more information.
For more help with citations see Citation Help.