This image looks like a real nightmare for edge detection etc and I don't know if you are really every going to get something satisfactory.
Do you need a robust automated method or is this just a once off?
If you need to do this a lot in future (for example if you want to collect images of samples for a scientific study) I would suggest photographing the leaf on a solid colour background - like a sheet of paper. That will allow you to do what you want much more easily, quickly and in a reliably automated way.
If this is a once off I would suggest (and this is going to sound like a joke but it isn't) using the 'remove background' feature in PowerPoint, or manually cutting the leaf out using software like MS paint (free), Inkscape (free) or Photoshop (not free).
Sorry can't be more help,