You don't have to prove anything, but there's no reason for me to believe that this isn't a fabricated claim either. People claim all sorts of crazy shit on the internet.

The Darth in the picture may or may not have been photoshopped. The words on the image may or may not have been uttered in that scene. And we don't know anything how the trial ended from this image.

Surely an event this outlandish (to the average person) was covered in the press somewhere?