How to remove the background from a sprite?

Question

Python

import pygame, random, os
weak = 1
strong = 2

class Enemy(pygame.sprite.Sprite):
    def __init__(self,disp,enemytype):
        pygame.init()
        self.screen = disp
        pygame.sprite.Sprite.__init__(self)
        if enemytype == weak:
            self.enemytype = weak
            self.image = pygame.image.load(os.path.join("images", "enemy1.png"))
            self.dx = 2.5
            self.hp = 1
        if enemytype == strong:
            self.enemytype = strong
            self.image = pygame.image.load(os.path.join("images", "enemy2.png"))
            self.dx = 5
            self.hp = 2
        self.image = self.image.convert()
        self.rect = self.image.get_rect()
        self.rect.left = self.screen.get_width()
        self.rect.centery = random.randrange(0, self.screen.get_height())
        self.moving = False

    def update(self):
        if self.moving == True:
            self.rect.centerx -= self.dx
            if self.rect.right < 0:
                self.reset()
        if self.hp == 0:
            self.reset()

    def reset(self):
        self.rect.left = self.screen.get_width()
        self.rect.centery = random.randrange(0, self.screen.get_height())
        self.moving = False
        if self.enemytype == weak:
            self.hp = 1
        elif self.enemytype == strong:
            self.hp = 2

The background is not removed after changing the picture