I don't want to use the instantly recognisable 'AI' engines or copy from a CD cover - are there resources with free-to-use images related to Gregorian Chant? It's for a poster advertising a chant workshop.
check the https://cantusdatabase.org database. Search for a well known (likely illuminated) chant from a holy day and see what pops up. Some archives permit free usage, others don't. It depends on their individual copyright terms, but with a little homework, I'm positive you'll find something nice here. Considering you would just be using it for a poster, and not for profit, you could probably use just about any graphic under "fair use".
The Cantus Database is not in the public domain, but its contents are published under the CC-BY-NC-SA, which might not be compatible with using it for an advertisement poster.
The Graduale Romanum from 1905 is in the public domain and it should be easy to obtain extant copies, because it has been replaced by most scholas with the Graduale Triplex. I have got my copy from a dissolved monastery and use it whenever I need a chant sample for a new video. It is not a manuscript, though, but a printed edition.
The cantus database merely links to other online resources. That’s why I mentioned that you’d need to check the terms of usage for each image, based on what the host archive’s usage policies. Cantus is just a meta index of chant databases.
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