Is it possible to change the way the SubGraphExpanderButton looks like.
I want, instead to press on the node itself in order to expand to group.
Can i bind a picture to this panel and replace the +/- sign ?
See the intro section on Button definitions. You can create your own button or adapt the existing SubGraphExpanderButton defined in Buttons.js.
Alternatively, you could define a click handler for your group template similar to the click function on the SubGraphExpanderButton.