Humanline is a nice resource for locating historic images. Humanline's galleries are arranged according to topic. You can browse the collections according to topic or search the collections by keyword. The topics are architecture, archeology, history, anthropology, people, lifestyle, religion, and engineering.
Humanline's images are available to educators and students for free re-use provided that the attribution guidelines are followed. There are two licensing agreements that users need to be aware of when attributing the works they're re-using. Those licenses are rights-managed and royalty-free. You do need to register as an education user to access all of the images for free.
Applications for Education
Humanline's galleries could be useful for developing history and art lessons. The images could be used by educators and students as visual aids in a presentation. The images could also be the starting point of a research project about a particular artist, topic, or theme in history.
H/T to David Kapuler.