The "Head of the Medusa" that inspired Shelley is now attributed to the Flemish School, circa 1620-30. The painting is owned by the Uffizi Gallery in Florence, inventory number P1472.
This image is taken from the Wikimedia Commons, which lists it as falling under the public domain in The United States, Australia, the European Union, and those countries with a copyright term of life of the author plus 70 years. Further, the official position taken by the Wikimedia Foundation is that "faithful reproductions of two-dimensional public domain works of art are public domain". This color image supplants the black and white copy from "Ekphrasis and the Other" by W. J. T. Mitchell from PICTURE THEORY (p. 174) published by The University of Chicago Press, 1994.