I see at least three common approaches museums are taking to images of their collections:
1. Maximum lockdown: no photos of the collection on the internet, or at most some very low-res ones on the museum's website. The physical museum itself will typically have anti-photography policies to try to enforce this. The goal is to de facto exercise exclusive rights to reproductions of the work (even where the copyright on the work itself has expired), as a revenue source, through e.g. high-quality art books, licensing of images, etc.
2. Disseminate through museum-owned channels. The museum digitizes its works and makes them available to the general public free of charge, via its own website, in at least fairly high-resolution images, a "virtual collection" that anyone can visit. Third-party dissemination may be possible in certain jurisdictions, but the museum either doesn't encourage or actively discourages it. The goal is to fulfill its public mission of dissemination/education, but while maintaining some control/stewardship of the work even online.
3. Maximum dissemination. The museum digitizes its works and makes them available in as many places as possible under a permissive license: its own website, archival repositories run by nonprofits and state institutions, Wikimedia, archive.org, news agency file-photo catalogues, etc. The goal is to fulfill its public mission of dissemination/education as widely as possible, and perhaps also achieve some advertising for the museum's collections and the works/artists it conserves, by ensuring that its works are the ones most likely to be used as illustrative examples in Wikipedia articles, books, newspaper/magazine articles, etc.