python的tkinter库的label使用

为什么我的窗口不能加载我下面这段代码中的图片（图片名称和格式都没错，也和代码在同一级文件夹内，程序不报错，能显示窗口，但是不能显示图片）：

self.label4=tk.Label(image=tk.PhotoImage(file='图标.png')) //该段代码在34行
self.label4.place(x=50,y=50)

全代码：

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

#连接数据库
def sql_link():
    conn=pymysql.connect(host='localhost',user='root',port=3306,db='testdb',charset='utf8',passwd='021018zxh')
    cur=conn.cursor()#生成游标对象
    sql="select* from employees"#sql语句
    cur.execute(sql)#执行sql语句
    data=cur.fetchall()#通过fetchall方法获得数据
    for i in data:
        print(i)
    cur.close()
    conn.close()

class MainWindow(tk.Tk):
    def __init__(self):
        super(MainWindow, self).__init__()
        self.title('我来上厕所')
        self.resizable(False,False)
        self.geometry('500x300+400+200')
        self.iconphoto(True,tk.PhotoImage(file='图标2.png'))
        self.loadIt()

        #登录界面
    def loadIt(self):
        self.label1=tk.Label(text='用户登录')
        self.label1.place(x=220,y=20)
        self.label2=tk.Label(text='账户:')
        self.label2.place(x=130,y=80)
        self.label3=tk.Label(text='密码:')
        self.label3.place(x=130,y=120)
        self.label4=tk.Label(image=tk.PhotoImage(file='图标.png'))
        self.label4.place(x=50,y=50)

        self.button1=tk.Button(text='管理员登录？点这试试',font=('宋体',8),fg='#A4A4A4',bd=0,command=None)
        self.button1.place(x=200,y=280)
        self.button2 = tk.Button(text='注册账户', font=('黑体', 8), fg='#A4A4A4', bd=0, command=self.load_register)
        self.button2.place(x=180, y=150)
        self.button3 = tk.Button(text='忘记密码', font=('黑体', 8), fg='#A4A4A4', bd=0, command=self.Retrieve_password)
        self.button3.place(x=280, y=150)
        self.button4 = tk.Button(text='登录', width=8,height=1,command=None)
        self.button4.place(x=220, y=200)

        self.txt1 = tk.StringVar()
        self.stxt1 = tk.Entry(self, textvariable=self.txt1)
        self.stxt1.place(x=180, y=80, width=170, height=20)
        self.txt2 = tk.StringVar()
        self.stxt2 = tk.Entry(self, textvariable=self.txt2,show='●')
        self.stxt2.place(x=180, y=120, width=170, height=20)

    #注册页面
    def load_register(self):
        window_sign_up=tk.Tk()
        window_sign_up.geometry('500x300+400+200')
        window_sign_up.wm_title('新用户')
        window_sign_up.resizable(False,False)
        window_sign_up.Rlabel1=tk.Label(window_sign_up,text='欢迎注册《我来上厕所》', font=('黑体', 12)).place(x=160, y=20)
        window_sign_up.Rlabel2=tk.Label(window_sign_up,text='给您一流的如厕体验~', font=('宋体', 8)).place(x=220, y=50)
        window_sign_up.Rlabel3=tk.Label(window_sign_up,text='设置昵称:').place(x=110, y=80)
        window_sign_up.Rlabel4=tk.Label(window_sign_up,text='设置密码:').place(x=110, y=120)
        window_sign_up.Rlabel5=tk.Label(window_sign_up,text='验证密码:').place(x=110, y=160)
        window_sign_up.Rtxt1 = tk.StringVar()
        window_sign_up.Rstxt1 = tk.Entry(window_sign_up, textvariable=window_sign_up.Rtxt1).place(x=180, y=80, width=170, height=20)  # 昵称输入
        window_sign_up.Rtxt2 = tk.StringVar()
        window_sign_up.Rstxt2 = tk.Entry(window_sign_up, textvariable=window_sign_up.Rtxt2, show='●').place(x=180, y=120, width=170, height=20)  # 密码输入
        window_sign_up.Rtxt3 = tk.StringVar()
        tk.Entry(window_sign_up, textvariable=window_sign_up.Rtxt3, show='●').place(x=180, y=160, width=170, height=20)  # 二次确认密码
        window_sign_up.Rbutton = tk.Button(window_sign_up,text='立即注册', width=8, height=1, command=None).place(x=220, y=200)

    #忘记密码页面
    def Retrieve_password(self):
        window_r_p=tk.Tk()
        window_r_p.geometry('500x300+400+200')
        window_r_p.wm_title('找回密码')
        window_r_p.resizable(False, False)
        window_r_p.Rlabel1 = tk.Label(window_r_p, text='┃找回密码', font=('黑体', 12)).place(x=100,y=60)
        window_r_p.Rlabel2 = tk.Label(window_r_p, text='请输入账号', font=('黑体', 10)).place(x=115,y=100)
        window_r_p.Rtxt1 = tk.StringVar()
        tk.Entry(window_r_p, textvariable=window_r_p.Rtxt1,background='#E0F2F7',relief="ridge").place(x=120, y=140, width=250, height=35)
        window_r_p.Rbutton1=tk.Button(window_r_p,text='立即找回',width=8,height=1,command=None).place(x=220,y=200)
        pass





if __name__=='__main__':
    mainwin=MainWindow()
    mainwin.mainloop()

程序中用到的两张图片如下：

img

img

必须在同一个文件夹内，而不是在同一级文件夹内
你先写个open来测试，看到底能不能打开文件，如果能找到文件，再说为什么不显示的问题，一步一步排查原因