Toggle by image on show-hide

Good day to you all,
I'm working on a code which is part of a menu.
That part of code is the Toggle show/hide function, with image.

I can right now make the show/hide toggle, but the image don't change from + to -.

How can I make my image change depending if Show or hide ?

function Toggle(li) {
var elem = document.getElementById(li);
if(!elem)return; // in case element doesn't exists in document;
elem.style.display = (elem.style.display != "none")? "none": "block";


}

Code (markup):

<img src="plus_icon.gif"  onclick="Toggle('Image_folder'); return false;" />

HTML:

Thanks !

 

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