Pygame Images Not Displaying At Correct Time

[Nether_1]

I have a simple pygame code that doesn't seem to be functioning correctly, check it out:

import pygame, time

active = True
screen_size = (1052, 700)

pygame.init()
DISPLAY = pygame.display.set_mode(screen_size)
firstimg = pygame.image.load('firstimage.png')
secondimg = pygame.image.load('secondimage.png')
clock = pygame.time.Clock()

def WHITESCREEN:
    DISPLAY.fill(255, 255, 255)
def DisplayFirstImg():
    DISPLAY.blit(firstimg, (1, 1))
def DisplaySecondImg():
    DISPLAY.blit(secondimg, (1, 1))
WHITESCREEN()
DisplayFirstImg()
time.sleep(3)
DisplaySecondImg()

while active:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            pygame.quit()
    pygame.display.update()
    clock.tick(15)

So this should display 'firstimage.png', wait three seconds, and then display 'secondimage.png'. However it skips the first two steps, going straight to the delay and then displays the second img. Obviously it shouldn't be doing this, so do you guys have any ideas? I would really appreciate it, and thank you in advance!