Library Collections & Photo Archives
Museum & Art Gallery Collections
For further information see http://www.metmuseum.org/research/image-resources/frequently-asked-questions.
Other Image Collections
When using material created by others, in teaching, publication or personal use (copying), there are many rights considerations. Here is a selection of resources, including Creative Commons licensed resources. These are usually free to use with some restrictions, such as crediting the creator (attribution).
Pics4Learning is a curated image library that is safe and free for education. Teachers and students can use the copyright-friendly photos and illustrations for classroom projects, web sites, videos, portfolios, or any other projects in an educational setting.
Search for whatever you're looking for
Under the search bar, click Settings -> Advanced search
At the bottom, click the dropdown menu for usage rights
Select an appropriate licence, e.g. free to use or share, even commercially, or free to use, share or modify, even commercially
Click the Advanced Search button