This image is a faithful representation of a medallion mosaic inside the Basilica of Saint Paul Outside the Walls which includes the portraits of all the Popes of the Catholic Church since the 1st century. For almost all of those mosaics, the authors are presumably unknown and the mosaics itself date back to circa 1850 (reconstruction of the basilica under the pontificate of Pius IX). As such they fall in the public domain. See popechart.com for documentation.
Note: This template should not be used for the images of the most recent medallions as they may still be protected by copyright.