I can't think of one centralized site where you can just scroll through and see things. Most museums or institutions will have at least part of their collections online (see the National Gallery, a different National Gallery, Smithsonian, MoMA, the Rijksmuseum for some examples). But the problem is many of those collections are absolutely huge, and digitizing objects takes a very long time and historically doesn't get done very thoroughly. Things are better than they used to be now that museums are cottoning on to the fact that people want to be able to browse objects without needing to go through traditional research channels, but it's inconsistent. Availability is basically contingent on whether someone down the line thought a work was important enough to properly catalogue.

If you want images that aren't as "public", you can browse auction results - there are a few databases for these, e.g. ArtNet, but most are subscription model if you want full details, and as the data is scrubbed directly from auctioneers the available info can be even more inconsistent.

I'm sure I'm missing something but who knows ¯\_(ツ)_/ ¯