Comprehensive reference work offers over 1,600 up-to-date entries on Islamic art and architecture ranging from the Middle East to Central and South Asia, Africa, and Europe and spans over a thousand years of history.
These websites list many sources of free public domain and creative commons images.
It's important to verify copyright status and allowed uses for each source you select.
An citation in MLA typically looks like this:
Artist or user name. Title. Date the image was created. Museum, City. Web. Database name or title of site. Date of access.
A citation in Chicago typically looks like this:
Artist's name. Title or description (image). Available from: Website or database. <URL>. (Accessed on date).