When you are working on a project for a class, you can include other people's images because it falls under part of copyright law know as Fair Use. Outside of your course work it is a different story. A couple of resources are listed below but for more information about copyright and the visual arts, you can look here.
Millions of primary source documents from across the world spanning the 15th - 21st centuries.
Many museums offer online access to their collections. Try these if you are looking for a particular work from a particular museum: