If I request a page about an image from Parsoid I get a 404. It doesn't automatically figure out to check Commons for it.
But somehow this works on the regular web site, e.g. via Media Viewer.
Example using Parsoid:
https://en.wikipedia.org/api/rest_v1/page/html/Thought
-> actual link is "./File:ThinkingMan_Rodin.jpg" or https://en.wikipedia.org/wiki/File:ThinkingMan_Rodin.jpg but if I didn't know better I would try to request that fromParsoid from the base wiki and then I get
-> https://en.wikipedia.org/api/rest_v1/page/html/File:ThinkingMan_Rodin.jpg --> 404
Using regular site:
https://en.wikipedia.org/wiki/Thought
-> actual link is pretty similar: <a href="/wiki/File:ThinkingMan_Rodin.jpg" class="image">
That one automatically has MediaView resolve it to
-> https://commons.wikimedia.org/wiki/File:ThinkingMan_Rodin.jpg --> 200
From @GWicke's comments on #mediawiki-parsoid it sound like this should be handled by RESTBase and not Parsoid.