[Tkinter] How do I change an image dynamically

[menator01]

#! /usr/bin/env python3

from tkinter import *
from tkinter import ttk

class Picture:
    def __init__(self, parent):
        self.parent = parent
        img = PhotoImage(file='img1.png')
        self.label = ttk.Label(self.parent)
        self.label['image'] = img
        img.image = img
        self.label.pack()

        btn = Button(self.parent, command=self.update, text='Test').pack(side='bottom', pady=50)

    def update(self):
        img = PhotoImage(file='img2.png')
        self.label['image'] = img
        img.image = img

def main():
    root = Tk()
    root.geometry('400x400+50+50')
    Picture(root)
    root.mainloop()

main()