When searching for images to use in your projects, it is important to understand who created the image and how it came to be online. Older works in the United States (created before 1923) are usually considered to be in the public domain, while newer works can be safe to use if the creator has made clear how they want to share them - either with Creative Commons licenses or waiving their copyright. Some images, while within copyright, can be safe to use within fair use, such as for a class project or presentation. But when copyright permission is required, give yourself plenty of time to research and request permission for publication.
ARTstor streamlines copyright clearance for specific institutions.
IAP: The Images for Academic Publishing (IAP) program makes available publication-quality images for use in scholarly publications free of charge. The IAP program was initiated by The Metropolitan Museum of Art in 2007 to help address the challenges of scholarly publishing in the digital age by providing free images for academic publications through an automated Web-based service. IAP was an optional service to all museums who wished to foster scholarly publications. This service is intended for individual scholars who are working on a particular book, article, or website that meets criteria established by the museum contributors. All IAP contributors also contributed images to the Artstor Digital Library, which makes images available exclusively for teaching and educational use, not publication.