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).convert_alpha()

clock = pygame.time.Clock()

x, y = 100., 100.
speed_x, speed_y = 133., 170.

while True:
    
    for event in pygame.event.get():
        if event.type == QUIT:
            exit()
    
    screen.blit(background, (0,0))
    screen.blit(sprite, (x, y))
    
    time_passed = clock.tick(30)
    time_passed_seconds = time_passed / 1000.0
    
    x += speed_x * time_passed_seconds
    y += speed_y * time_passed_seconds    

    # If the sprite goes off the edge of the screen,
    # make it move in the opposite direction
    if x > 640 - sprite.get_width():
        speed_x = -speed_x
        x = 640 - sprite.get_width()
    elif x < 0:
        speed_x = -speed_x
        x = 0.
        
    if y > 480 - sprite.get_height():
        speed_y = -speed_y
        y = 480 - sprite.get_height()
    elif y < 0:
        speed_y = -speed_y
        y = 0
        
    pygame.display.update()