i remember that 20 years ago (I’m sure more), the Open Access movement (OA) began with a focus on remove the pricing barriers on paid academic journals. Around the same time, academic researchers I knew started putting their research papers on their own websites for free. The whole idea behind Open Access is to make materials available for public consumption.
For images, free material can be found in the royalty-free, public domain, copyright-expired, and Creative Commons (and other “free” licenses) spaces. This makes images available for public use. This use can even be for commercial purposes in most cases.
I remember over a decade ago when the museums started releasing classical art images into the public domain in the form of Open Access databases. The image banks below just tip the iceberg of the many, many resources out there.
Smithsonian Open Access is a free, downloading image bank with millions of items you can use here:
https://www.si.edu/openaccess. Read this article about the release of images, https://www.smithsonianmag.com/smithsonian-institution/smithsonian-releases-28-million-images-public-domain-180974263/.
Open Society Foundations has a nice page on Open Access here – https://www.opensocietyfoundations.org/explainers/what-open-access
Art Institute Chicago has a great Open Access page for free images here: https://www.artic.edu/open-access/open-access-images. They even have a public API page of projects that tap into their online collection through other programs: https://www.artic.edu/open-access/public-api.
National Gallery of Art (NGA) has an open access page with 50K+ free, downloadable images here:
https://www.nga.gov/open-access-images.html. They also have an open data page (https://www.nga.gov/open-access-images/open-data.html) where you can download a CSV (UTF-8 encoded) file of data (this is metadata) about 130K+ artworks. The file is located at Github, a repository for free data, programs, etc. https://github.com/NationalGalleryOfArt/opendata
Other resources
Getty – https://www.getty.edu/projects/open-content-program/
MOMA (UK) – https://www.moma.co.uk/public-domain-images/
OA at Cleveland MOA – https://www.clevelandart.org/open-access
Open Access at MOMA – https://www.metmuseum.org/about-the-met/policies-and-documents/open-access
Pratt University Libguide – https://libguides.pratt.edu/c.php?g=763369&p=8238035
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