Error with variables in different functions and scripts [on hold]

My friend and I are making a game in Pygame for fun. I was trying to clean up the code by putting more things into separate scripts as functions. This is the main script, and the only function is for loading some images.

#set up
import pygame, sys, random, time, math, imageL
from pygame.locals import *
pygame.init()

#variables start----------------------------------
imageL.load_image

x, y = 0, 0 #character position
movex, movey = 0, 0 #how far the character will move
#x is left and right, y is up and down

z, w = random.randint(10, 480), random.randint(10, 500)
movez, movew = 0, 0


#variables end------------------------------------

screen = pygame.display.set_mode((850, 640),0,32) #set screen                       
background = pygame.image.load(niass).convert() #load image to screen


char = pygame.image.load(mil).convert_alpha() #covert player image
ali = pygame.image.load(ali_shit).convert_alpha() #covert alien image


while True:

    for event in pygame.event.get():
            if event.type==QUIT:
                pygame.quit()
                sys.exit()
    if event.type==KEYDOWN:    
            if event.key==K_a:
                    movex=-1
            elif event.key==K_d:
                    movex=+1
            elif event.key==K_w:
                    movey=-1
            elif event.key==K_s:
                    movey=+1
    if event.type==KEYUP:
            if event.key==K_a:
                    movex=0
            elif event.key==K_d:
                    movex=0
            elif event.key==K_w:
                    movey=0
            elif event.key==K_s:
                    movey=0

    if z < x:
        movez =+ 0.20
    elif z > x:
        movez =- 0.20
    if w < y:
        movew =+ 0.20
    elif w > y:
        movew =- 0.20


    x += movex
    y += movey
    w += movew
    z += movez


    screen.blit(background,(0,0))
    screen.blit(char,(x,y))
    screen.blit(ali,(z,w))

    pygame.display.update()

This is the script with the load image function:

def load_image():
    niass = "grass.png" #grass image
    milf = "head.png" #player image
    ali_shit = "ahead_2.png" #alien image

then this is the error i get when i run this

>>> 

Traceback (most recent call last):
  File "/home/claude/Dropbox/BigKahunaBurger/BigKahunaBurger LOOP.py", line 24, in <module>
    background = pygame.image.load(niass).convert() #load image to screen
NameError: name 'niass' is not defined
>>> 

I've heard of global variables, but I don't know how to use them.

Any help would be greatly appreciated.