> Is there a particular reason that  schema:contentUrl  was chosen to point to URLs of the form <https://upload.wikimedia.org/wikipedia/commons/q/q1/filename.jpg> rather than <http://commons.wikimedia.org/wiki/Special:FilePath/filename.jpg> that WDQS uses?
> Is the specific location useful?  (Is it even stable?)  Or would it make more sense to use the established url form for these links, as used in WDQS ?

But, anyway, it should be fairly easy to change them to point to
Special:FilePath instead and, indeed, make interoperability with
Wikidata easier.
I could easily write a patch for it if the Search team is ok with it.
May you open a Phabricator ticket about it?