Background помогите заменить задний фон на картинку

Попробуйте так:

import os
import sys
# ??? os.system('pip install pygame')
import pygame
from pygame import *


class TextInputBox(pygame.sprite.Sprite):
    def __init__(self, x, y, w, font):
        super().__init__()
        self.color = (0, 255, 179)
        self.backcolor = None
        self.pos = (x, y) 
        self.width = w
        self.font = font
        self.active = False
        self.text = ""
        self.render_text()

    def render_text(self):
        t_surf = self.font.render(self.text, True, self.color, self.backcolor)
        self.image = pygame.Surface(
            (max(self.width, t_surf.get_width()+10), t_surf.get_height()+10), 
            pygame.SRCALPHA
        )
        if self.backcolor:
            self.image.fill(self.backcolor)
        self.image.blit(t_surf, (5, 5))
        pygame.draw.rect(self.image, self.color, self.image.get_rect().inflate(-2, -2), 2)
        self.rect = self.image.get_rect(topleft = self.pos)

    def update(self, event_list):
        for event in event_list:
            if event.type == pygame.MOUSEBUTTONDOWN and not self.active:
                self.active = self.rect.collidepoint(event.pos)
            if event.type == pygame.KEYDOWN and self.active:
                if event.key == pygame.K_RETURN:
                    self.active = False
                elif event.key == pygame.K_BACKSPACE:
                    self.text = self.text[:-1]
                else:
                    self.text += event.unicode
                self.render_text()

width = 1000
height = 600

pygame.init()
window = pygame.display.set_mode((width, height))

# +++ vvv
background = pygame.image.load("background.jpg")                  # !!! установите свое .jpg
background = pygame.transform.scale(background, (width, height)) 
all_sprites = pygame.sprite.Group()
# +++ ^^^

clock = pygame.time.Clock()
font = pygame.font.SysFont(None, 100)

text_input_box = TextInputBox(50, 50, 400, font)
group = pygame.sprite.Group(text_input_box)

all_sprites.add(text_input_box)

run = True
while run:
    clock.tick(120)
    event_list = pygame.event.get()
    for event in event_list:
        if event.type == pygame.QUIT:
            run = False
    group.update(event_list)

    window.blit(background,(0,0))                                            # !!!   
    all_sprites.draw(window)                                                 # !!!    
    pygame.display.update()                                                  # !!! 

    '''
    window.fill(0)
    group.draw(window)
    pygame.display.flip()
    '''
    

pygame.quit()
exit()