pygame windowが真っ白になる

pygame windowが真っ白で何も表示されないのですがどうすればよいでしょうか。

python
1import sys
2from random import randint 
3import pygame
4from pygame.locals import QUIT,Rect,KEYDOWN,K_SPACE
5
6pygame.init() #初期化
7pygame.key.set_repeat(5,5)
8SURFACE=pygame.display.set_mode((800,600))
9FPSCLOCK=pygame.time.Clock()
10
11def main():
12    #メインルーチン
13    walls=80
14    ship_y = 250
15    velocity=0
16    score=0
17    slope=randint(1,6)
18    sysfont =pygame.font.SysFont(None,36)
19    ship_image=pygame.image.load("yossi.jpeg")
20    bang_image=pygame.image.load("expro.jpg")
21    holes=[]
22    for xpos in range(walls):
23        holes.append(Rect(xpos*10,100,10,400))
24    game_over = False
25
26    while True:
27        is_space_down = False
28        for event in pygame.event.get():
29
30            if event.type == QUIT:
31                pygame.quit()
32                sys.exit()
33            elif event.type == KEYDOWN:
34                if event.key == K_SPACE:
35                    is_space_down = True
36
37        #自機の移動
38        if not game_over:
39            score += 10
40            velocity += -3 if is_space_down else 3 
41            ship_y += velocity
42
43        #洞窟をスクロール
44        edge=holes[walls-1].copy()
45
46        test = edge.move(0,slope)
47        if test.top<=0 or test.bottom>=600:
48            slope = randint(1,6)*(-1 if slope>0 else 1)
49            edge.inflate_ip(0,-20)
50        edge.move_ip(10,slope)
51        holes.append(edge)
52        del holes[0]
53        holes = [x.move(-10,0)for x in holes]
54
55        #衝突？
56        if holes[0].top > ship_y or holes[0].bottom < ship_y + 15: 
57            game_over = True
58
59    #描画
60    SURFACE.fill((255,255,255))
61    for hole in holes:
62        pygame.draw.rect(SURFACE,(0,0,0),hole)
63    SURFACE.blit(ship_image,(0,ship_y))
64    score_image=sysfont.render("score is %d"%(score),True,(0,0,225))
65    SURFACE.blit(score_image,(600,20))
66
67    if game_over:
68        SURFACE.blit(bang_image,(0,ship_y-40))
69
70
71    pygame.display.update()
72    FPSCLOCK.tick(15)
73
74if __name__ == '__main__':
75    main()