This is the basic screen blocker with a background image.

screen_blocker.py

from PIL import Image, ImageTk

try:
	import Tkinter as tk
except ImportError:
	import tkinter as tk

try:
	import ttk
except ImportError:
	import tkinter.ttk as ttk

class FullScreenBlocker(tk.Tk):
	def __init__(self):
		tk.Tk.__init__(self)
		self.top=self
		# no border; full screen
		self.top.attributes('-fullscreen', 'true')
		
		# always on top
		self.top.attributes('-topmost', 'true')
		
		self.geometry("{0}x{1}+0+0".format(self.winfo_screenwidth(), self.winfo_screenheight()))
		
		w,h = self.winfo_screenwidth(),self.winfo_screenheight()
		cv = tk.Canvas(self, width=w, height=h)
		cv.pack(fill='both', expand='yes')
		pil_image = Image.open("this_is_my_background.gif")# you can use .gif, .png, .jpg..
		pil_image2 = pil_image.resize((w,h), Image.ANTIALIAS)   
		tk_image = ImageTk.PhotoImage(pil_image2)
		cv.create_image(w//2, h//2, image=tk_image, anchor='center')
		tk.Button(cv, text="kill me!", command=self.destroy).pack()
		
		self.mainloop()
		
	def _show_clnt_confg(self):
		clnt_top = tk.Toplevel(self.top)
		# always on top
		clnt_top.attributes('-topmost', 'true')
		top_ClntLogin = ClntLogin(clnt_top)

fscrnblcker = FullScreenBlocker()