background_image_filename = 'sushiplate.jpg'
sprite_image_filename = 'fugu.png'

import pygame
from pygame.locals import *
from sys import exit

pygame.init()

screen = pygame.display.set_mode((640, 480), 0, 32)

background = pygame.image.load(background_image_filename).convert()
sprite = pygame.image.load(sprite_image_filename)

# Our clock object
clock = pygame.time.Clock()

x1 = 0.
x2 = 0.
# Speed in pixels per second
speed = 250.

frame_no = 0

while True:
    
    for event in pygame.event.get():
        if event.type == QUIT:
            exit()
    
    screen.blit(background, (0,0))
    screen.blit(sprite, (x1, 50))
    screen.blit(sprite, (x2, 250))
    
    time_passed = clock.tick()
    time_passed_seconds = time_passed / 1000.0
    
    distance_moved = time_passed_seconds * speed
    x1 += distance_moved
    
    if (frame_no % 5) == 0:
        distance_moved = time_passed_seconds * speed
        x2 += distance_moved * 5.
    
    # If the image goes off the end of the screen, move it back
    if x1 > 640.:
        x1 -= 640.    
    if x2 > 640.:
        x2 -= 640.  
        
    pygame.display.update()
    frame_no += 1