I scraped a few thousand images from an open source online collection (https://www.metmuseum.org/art/collection --> Open Access Artworks) for a coding project. I want to cite the selected works from the collection with one Harvard style citation for my code report's bibliography. None of them show up in the report itself, they're just research materials. Does anyone have a suggestion for this?
In this case it turns out that the Metropolitan Museum has made available a large number of public-domain works under the Creative Commons Zero license. Legally, I think this means you don't need to cite anything, but you'll probably still want to, both in order to be a good scholar and to encourage the museum to keep expanding their digital collection.
Individually citing thousands of individual images is obviously excessive, especially when they don't show up in the paper. Instead, I would suggest citing one or more of the datasets they have made available on GitHub. You might not have used these directly, but if they include all the images you've used this makes for an inclusive reference. Also, they write
Please consider attributing or citing The Metropolitan Museum of Art's CC0 select datasets, especially with respect to research or publication. Attribution supports efforts to release other datasets in the future. It also reduces the amount of "orphaned data," helping to retain source links.
For reproducibility reasons, you might also want to include a list of all the pictures (or their accession codes) in some kind of electronic Supplemental Material.
For completeness, if you want to cite an individual image, their FAQ states
How should I cite and credit a CC0 image that I've downloaded from The Met's website?
CC0 (Creative Commons Zero) does not require any attribution or credit, but as an educational institution, the Museum hopes that you will include a citation. All the information necessary for proper citation of a CC0 image is available on the Collection page for that object.
A sample object credit line format is as follows: "The Metropolitan Museum of Art, New York," followed by the specific information identified as the "Credit Line" on the Collection page for each work of art. The citation could also include the URL www.metmuseum.org.