error in coding in pygame

You have to call pygame.display.flip() whenever you want the screen to be updated.

As well as calling pygame.display.flip() (or pygame.display.update() if it is a software based surface). you will also need to call pygame.init() at the start of your program and eventually call pygame.quit() to close down the window and do cleanup.

you code might then be

import pygame,time
from pygame.locals import *
pygame.init()
screen=pygame.display.set_mode()
nin=pygame.image.load('/home/satyajit/Desktop/nincompoop0001.bmp')
screen.blit(nin,(50,100))
pygame.display.flip()
time.sleep(10)
pygame.quit()

Try this:

import pygame
from pygame.locals import *
screen=pygame.display.set_mode()
nin=pygame.image.load('nincompoop0001.bmp')
screen.blit(nin,(50,100))

Then, place the image in the same directory as your script.