Bottom of the transparent button on tkinter

I have some buttons with an image and they have a standard gray background that I can not leave transaparente. The goal is to leave only the image of the button and behind it has the background of the window (which is a label). How do I make the background button transparent?

I use a function to build the button follows it:

def make_button_img(self, w, h, url, func, pos, col,row,alinhamento):
    self.arq = Image.open(url).resize((w, h), Image.ANTIALIAS)
    self.img.append(ImageTk.PhotoImage(self.arq))
    btn = Button(self, image=self.img[pos],activeforeground="Green",relief='flat', height=h, width=w, command=func)


    btn.grid(column=col, row=row,pady=15,sticky=alinhamento)
    return btn