You probably know more about copyright law than I do given your studies/thesis.  I am not a lawyer, I  am only offering my opinions based on my reading.   In cases other than a photo of an existing copyright protected piece of art the "content" of a photograph is not really relevant insofar as being judged a "work of art".  The copyright law is there to protect the rights of the photographer to the image captured.  A photographer who captures the assassination of a political figure, the knockout punch in a historic boxing match or the latest escapades of Paris Hilton has rights to the image.  None of these would count as a work of art.  Art critics have declared a piece of art a work of genius only to find it was painted by a chimpanzee so art is completely subjective but none of that changes the right to copyright protection.  Although I don't know if a chimp can own the rights to its painting:)

In your example of the photos of the works of art, I would suggest that it is not the fact that it was a photograph but that the "image" did not contain any originality.  If I painted the Mona Lisa as a reproduction then I would expect that the ruling would have been the same.

There is nothing wrong in using photos as inspiration but in relation to copyright infringement I don't think that it matters the "medium" if the image is recreated/repainted/reproduced so that it looks like the original.  Many matte paintings that are used in movies begin as a photo that gets manipulated, painted over, then has additional cgi elements and light effects added in.  At certain stages of the matte the original source material may still be recognizable but in the end the image bears no resemblance to the source images.

The question you have to ask yourself is if someone looked on the source/reference photo and your painting do they see substantial similarities between the two images?

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