This is the question that a 1709 Blog friend writing under the nom de plume of Leonardo [of course: what else?] wishes to pose to this Blog's readers.
"I can see" - continues Leonardo - "that under say: US copyright law, one might argue that the altered painting was a derivative work, hence the drawing of the moustache would be arguably an infringing preparation of a derivative work. But the adaptation right under the Copyright Designs Patents Act 1988 (CDPA) does not extend to artistic works. So, it might be vandalism, it might be an infringement of the artist's moral rights, but I can't see how it would be an infringement of copyright per se. Is there something I'm missing?"
Among other things, Section 21(1) CDPA provides that "The making of an adaptation of the work is an act restricted by the copyright in a literary, dramatic or musical work", thus seemingly excluding artistic works from its scope.
As such, this blogger reckons that Leonardo may be indeed right. If anything copyright-related, could this be just a case of moral rights (notably: right of integrity) infringement?