why is the picture not displayed on the pygame window? the code does not give errors

Question

import pygame    
import sys    
from pygame.locals import *    
import random    
import time    
GREY = pygame.Color(128, 128, 128)        
RED = pygame.Color(255, 0, 0)    
BLUE = pygame.Color(0, 0, 255)    
GREEN = pygame.Color(0, 255, 0)    
BLACK = pygame.Color(0, 0, 0)    
WHITE = pygame.Color(255, 255, 255)    
PINK = pygame.Color(255, 192, 203)    
FPS = 60     
FramePerSec = pygame.time.Clock()    
screen = pygame.display.set_mode((1024, 526))    
screen.fill(GREY)    
to_up = False    
to_down = False    
speed_gg_up = 132    
speed_gg_down= 56    
gg = pygame.image.load('gg.png')    
gg_x = 56    
gg_y = 264    
while True:    
    for event in pygame.event.get():    
        if event.type == pygame.KEYDOWN:    
            if event.key== pygame.K_UP:    
                to_up = True    
            if event.key == pygame.K_DOWN:    
                to_down = True    
        if event.type == QUIT:    
            pygame.quit()    
            sys.exit()    
    FramePerSec.tick(FPS)    
    if to_up:    
        gg_y -= speed_gg_up    
        time.sleep(2)    
        gg_y += speed_gg_up    
        to_up = False    
    if to_down:    
        gg_y +=speed_gg_down    
        time.sleep(2)     
        gg_y -= speed_gg_down    
        to_down = False    
    screen.blit(gg, (gg_x, gg_y))     
    pygame.display.update()    

the code works without errors, but gives only an empty window. no matter how much I change the code, nothing changes please help me find and fix the error so that the code starts showing the image