python tkinter display current time in the center

i am starting with GUI in python with tkinter.and i want to display current time as HH:MM:SS in the center of my windows(with big digit if possible).this is the app code: a basic full screen windows with a background picture.

import Tkinter as tk
from Tkinter import Tk, Frame, BOTH
import Tkinter
from PIL import Image, ImageTk
root = tk.Tk()
root.attributes('-fullscreen', 1)
im = Image.open('spring.png')
tkimage = ImageTk.PhotoImage(im)
myvar=Tkinter.Label(root,image = tkimage)
myvar.place(x=0, y=0, relwidth=1, relheight=1)
root.mainloop()

EDIT: by current time i mean to keep updating the time as a clock