Once you do that you should see an alert each time an image is added to the canvas. This is the first step to the process. Once that is confirmed the next step well to to detect facebook image and apply the roudned corners via the fabric image api. I haven’t quite worked out the details on that piece of it yet.
For anyone else who wants to help the fabricImage variable will be an instance of this.
Perhaps fabric has a dedicated forum where you can specifically ask how to apply rounded corners to an instance of fabric.Image.
It seems to add a class to the body tag, but that’s only useful for styling the page the canvas appears on, rather than the canvas elements themselves.
Unless you present the actual page where we can see everything in action it is hard to say what could be the issue. Having said that are there any errors in the console?