Problems with overlapping images in pygame

Question

I'm having problems with blitting images to rect objects in pygame. I have a background image blitted to my main pygame window and also an image blitted on a rect object on the screen which moves. The problem is taht the rect object is overlapping my background image when it's moving around. I was looking for only be able to see the green helicopter shape and not the black outline around it.

The .convert_alpha() worked for my helicopter image to remove the rectangle outline. I'm having the same problem with my missile moving wall objects now. Where I can see the rect object outline around my missiles.

import pygame as pg
import random as r
import time
import openpyxl as op
pg.init()


def load_best ():
    file = 'best.xlsx'
    wb= op.load_workbook(file)
    sheet = wb['Sheet1']
    best = sheet.cell(row=1, column=1).value
    return best
    
def save_best (score):
    file = 'best.xlsx'
    wb= op.load_workbook(file)
    sheet = wb['Sheet1']
    sheet['a1'] = score
    wb.save(file)
    
    
MAX_X = 1190
MAX_Y = 590
MIN_X = 10
MIN_Y = 10
SIZE = 100
SPEED = 2
COLOR = (255,145,0)
move_amount = 0
MISSILE_THICK = 20



wn = pg.display.set_mode((1200, 600))
#### only heli should be convert_alpha #################
BG_IMG = pg.image.load('desert.png').convert()
BG_IMG = pg.transform.scale(BG_IMG, (1200, 600))
MOVING_WALL_IMG = pg.transform.scale(pg.image.load('missile.png').convert_alpha(), (SIZE, MISSILE_THICK))
STILL_WALL_IMG = pg.transform.scale(pg.image.load('wall.gif').convert(), (MAX_X, 10))
                              
class Wall (pg.Rect):
    def __init__(self, posX, posY):
        self.xcor = posX
        self.ycor = posY
        self.rect = None

class Heli (pg.Rect):
    def __init__(self, posX, posY):
        self.image = pg.transform.scale(pg.image.load('li.png').convert_alpha(), (150,100))
        self.rect = self.image.get_rect()
        
        self.rect.x = posX
        self.rect.y = posY
        

# top and bottom constant walls
TOP = pg.Rect(MIN_X, MIN_Y-10, MAX_X, 10)
BOTTOM = pg.Rect(MIN_X, MAX_Y, MAX_X, 10)


heli = Heli(MIN_X + 100, MAX_Y //2)


# keep moving walls in a list
moving_walls = [Wall(MAX_X, r.randint((MIN_Y + 10), (MAX_Y - 10)))]


# count is a control for how many iterations of the main loop have past
count = 0

best = load_best()
# main loop
while True:
    count += 1
    # fill screen
    wn.fill('black')

    # show score text on screen
    font = pg.font.Font('freesansbold.ttf', 32)
    text = font.render(f' Score : {count//2}', True, 'cyan')
    textRect = text.get_rect()
    textRect.center = (100,50)

    # show best score text on screen
    best_font = pg.font.Font('freesansbold.ttf', 32)
    best_text = best_font.render(f' Best : {best}', True, 'cyan')
    best_textRect = best_text.get_rect()
    best_textRect.center = (100,100)
    #wn.blit(text, textRect)
    

    # editing objects to move

    

    # blitting must happen before everything else
    pg.draw.rect(wn,COLOR, heli.rect)
    
    wn.blit(BG_IMG, (0,0))
    
    wn.blit(heli.image, heli.rect)
    wn.blit(text, textRect)
    wn.blit(best_text, best_textRect)

    heli.rect.y += move_amount
    heli.rect.y += 1


    

    # dealing with the moving walls and their conditions
    for wall in moving_walls :
        wall.rect = pg.Rect(wall.xcor, wall.ycor, SIZE, MISSILE_THICK)
        #wall.rect = MOVING_WALL_IMG.get_rect()
        pg.draw.rect(wn, COLOR, wall.rect)
        wn.blit(MOVING_WALL_IMG, wall.rect)
        wall.xcor -= SPEED

        if wall.xcor < MIN_X + 10:
            moving_walls.remove(wall)


        # COLLISIONS
        collide = [pg.Rect.colliderect(heli.rect, wall.rect), pg.Rect.colliderect(heli.rect, BOTTOM), pg.Rect.colliderect(heli.rect, TOP)]
        if collide[0] or collide[1] or collide[2] :
            # taking care of best score when collided
            best = load_best()
            if count//2 > best:
                save_best(count//2)
            time.sleep(2)
            pg.quit()
            


    # THIS Part controls the speed at which moving_walls  respawn       
    ## LEVEL 1
    if count < 3000 :
        
    
        if count%250 == 0 :
            moving_walls.append(Wall(MAX_X, r.randint((MIN_Y + 10), (MAX_Y - 10))))
            
            
    ## LEVEL 2
    if count > 3000 and count < 6000 :
        
    # count controls frequency of wall spawns
        if count%150 == 0 :
            moving_walls.append(Wall(MAX_X, r.randint((MIN_Y + 10), (MAX_Y - 10))))

    ## LEVEL 2
    if count > 6000 and count < 1000000 :
        
    # count controls frequency of wall spawns
        if count%50 == 0 :
            moving_walls.append(Wall(MAX_X, r.randint((MIN_Y + 10), (MAX_Y - 10))))
            


        
    # drawing all objects back to the screen
    

    
    pg.draw.rect(wn, COLOR, TOP)
    pg.draw.rect(wn, COLOR, BOTTOM)
    wn.blit(STILL_WALL_IMG, TOP)
    wn.blit(STILL_WALL_IMG, BOTTOM)
    #pg.draw.rect(wn,COLOR, heli.rect)
    
    # check for collisions
    collide = pg.Rect.colliderect(heli.rect, wall.rect)

    # update window
    pg.display.update()

    # event handling
    for ev in pg.event.get():

        # key presses
        if ev.type == pg.KEYDOWN:
            if ev.key == pg.K_SPACE:
                move_amount = -3
        if ev.type == pg.KEYUP:
            move_amount = 0
            
        # quit button        
        if ev.type == pg.QUIT:
            pg.quit()
    time.sleep(0.01)