The ruling's section on transformativeness explains the distinction. Note that "derivative works" under US copyright law works differently from how it gets defined in typical open source licenses.

My understanding is that, for the purposes of determining fair use, a derivative work is substantially the same thing but in a different format. Transformative work must involve significant additional creative contribution "Changing the medium of a work is a derivative use rather than a transformative one." They cite previous case law that holds repackaging a print book as an e-book as a "paradigmatic example of a derivative work." The law also offers some paradigmatic examples of transformative work, such as criticism, commentary and scholarship.

Based on all of that, I would guess that, for the purposes of copyright law, a JPEG of a painting is absolutely a derivative work and not a transformative one.