Moving picture to random location. Pygame

How do I remove the previous picture placed. So that only one picture is on the screen at once

import pygame
from random import randint
pygame.init()
ekraan = pygame.display.set_mode([640, 480])
ekraan.fill([255,255,255])
tootab = True

waldo = pygame.image.load("wwaldo.jpg")
ekraan.blit(waldo, (50,50))




pygame.display.flip()

while tootab:
    for i in pygame.event.get():
        if i.type == pygame.QUIT:
            tootab = False

        elif i.type == pygame.MOUSEBUTTONUP:
            x = randint(0,640)
            y = randint(0,480)
            ekraan.blit(waldo, (x,y))
           pygame.display.flip()

pygame.quit()