新手写了一段程序，显示图片，程序不报错，但显示不出图片，不知道为什么，请教！

import tkinter as tk
from tkinter import *
from PIL import Image,ImageTk

class pic(Canvas):
    i=0
    j=0
    n=0
    def __init__(self,fr,i,j):
        Canvas.__init__(self,fr)
        self.i=i
        self.j=j
        self.put(0,0)
    def put(self,bw,n):
        img0=Image.open("0.jpg")
        img1=Image.open("1.jpg")
        img2=Image.open("2.jpg")
        im0=ImageTk.PhotoImage(img0)
        im1=ImageTk.PhotoImage(img1)
        im2=ImageTk.PhotoImage(img2)
        self.create_image(300,200,image=im0)
frame=tk.Tk()
b1=pic(frame,0,0)
b1.pack()
frame.mainloop()