How to scale images to screen size in Pygame

You can scale the image with pygame.transform.scale:

import pygame
picture = pygame.image.load(filename)
picture = pygame.transform.scale(picture, (1280, 720))

You can then get the bounding rectangle of picture with

rect = picture.get_rect()

and move the picture with

rect = rect.move((x, y))
screen.blit(picture, rect)

where screen was set with something like

screen = pygame.display.set_mode((1600, 900))

To allow your widgets to adjust to various screen sizes,
you could make the display
resizable:

import os
import pygame
from pygame.locals import *

pygame.init()
screen = pygame.display.set_mode((500, 500), HWSURFACE | DOUBLEBUF | RESIZABLE)
pic = pygame.image.load("image.png")
screen.blit(pygame.transform.scale(pic, (500, 500)), (0, 0))
pygame.display.flip()
while True:
    pygame.event.pump()
    event = pygame.event.wait()
    if event.type == QUIT:
        pygame.display.quit()
    elif event.type == VIDEORESIZE:
        screen = pygame.display.set_mode(
            event.dict['size'], HWSURFACE | DOUBLEBUF | RESIZABLE)
        screen.blit(pygame.transform.scale(pic, event.dict['size']), (0, 0))
        pygame.display.flip()