It was in memoriam to Gaston Julia’s Birthday.
In case I need to precise, the fractals you see on the logo are called Julia sets because the French mathematician Gaston Julia described them first. However, most of my readers already know that, right? ;-)
To say something only initiated people can understand: “The Mandelbrot set contains all Julia sets”. (That is why the fractal on the left is actually the Mandelbrot set.)