NEWBIE HELP: How to tell Python when to use a function

[snippsat]

A simple demo that is close to what you want.

#rand_pick.py
import webbrowser
import random
import os

def rand_numb():
   numb = random.randrange(1, 6)
   return numb

def pics(numb):
   for img in os.listdir():
       if numb in img:
           webbrowser.open(img)

if __name__ == '__main__':
   numb = str(rand_numb())
   pics(numb)

So in folder 5 images(pic_1.jpg, pic_2.jpg..ect).
No path is given so you have the script rand_pick.py in same folder as images.
Will random pick 1 image,and show it it browser or OS image viewer.