Python HELP ?!

I am very new to python. How do I located a file on my computer (Mac) in a python program. Here is the program I am trying to run (I know its not done yet):

bif="bg.jpg"
mif="ball.png"

import pygame, sys
from pygame.locals import *

pygame.init()
screen=pygame.display.set_mode
((420,280),0,32)

backround=
pygame.image.load(bif).convert()

mouse_c=
pygame.image.load(mif).convert_alpha()

while True:
for event in pygame.event.get():
if event.type == QUIT:
pygame.quit()
sys.exit()

screen.blit(backround, (0,0))


This is the error I get when I try to run it:

Traceback (most recent call last):
File "/Users/Ben/Desktop/Python/Game", line 10,
in
backround=pygame.image.load(bif).convert()
error: Couldn't open bg.jpg


How to put in the location (on my computer) for "bif" and "mif"?

Keep in mind I am using a mac.


Thanks,
Ben