convert_alpha不生效

qhlouc

import pygame
import sys
from pygame.locals import *
from random import *

class Ball(pygame.sprite.Sprite):
    def __init__(self, image, pos, speed, bg_size):
        pygame.sprite.Sprite.__init__(self)

        self.image = pygame.image.load(image).convert_alpha()
        self.rect = self.image.get_rect()
        self.rect.left, self.rect.top = pos
        self.speed = speed
        self.bg_width, self.bg_height = bg_size[0], bg_size[1]

    def move(self):
        self.rect = self.rect.move(self.speed)

        if self.rect.right < 0:
            self.rect.left = self.bg_width    # 穿越
        elif self.rect.left > self.bg_width:
            self.rect.right = 0
        elif self.rect.bottom < 0:
            self.rect.top = self.bg_height
        elif self.rect.top > self.bg_height:
            self.rect.bottom = 0

def main():
    pygame.init()

    ball_image = "gray_ball.png"
    bg_image = "background.png"

    running = True

    bg_size = width, height = 1024, 681
    radius = 100
    clock = pygame.time.Clock()

    screen = pygame.display.set_mode(bg_size)
    pygame.display.set_caption("Play the ball")

    background = pygame.image.load(bg_image).convert_alpha()

    balls = []
    for i in range(5):
        pos = randint(0, width - radius), randint(0, height - radius)
        speed = [randint(-10, 10), randint(-10, 10)]
        ball = Ball(ball_image, pos, speed, bg_size)
        balls.append(ball)

    while running:
        for event in pygame.event.get():
            if event.type == QUIT:
                sys.exit()
        
        screen.blit(background, (0, 0))

        for ball in balls:
            ball.move()
            screen.blit(ball.image, ball.rect)

        pygame.display.flip()
        clock.tick(30)


if __name__ == "__main__":
    main()

在学习小甲鱼的python入门课程，ball图片已经使用convert_alpha转换，但出来的效果仍然有黑色背景边框。
请大家帮忙看看是哪里出了问题？

低调的恶棍

兄弟，我花了两个小时终于解决了这个问题，需要用ps把图片的背景去掉并转化为png格式才能用