Hello super joomlers!
After searching the web, the official website, php documentation and forums, I found, at last, the solution thanks to a friend's suggestion : "What if it was just the image background?"
Sure enough, by analysing the code of Imagick library using my favourite PHP IDE PhpStorm, I discovered the right method to call just after the Imagick constructor. Here is some sample code.
$image = new Imagick('example.png');
It's your turn now! Happy coding!