[Zope] Returning a random image from a folder

[David Mitchell]

Going mad with this one...

I'm trying to write a Python script to return a random image from a specified 
folder.  For example, here's how I may want to call it from a ZPT page:

<span tal:replace="randomImage('Advertisements/BannerAds') / >

My randomImage Python script is as follows (where 'directory' is the parameter 
being passed to the script):

  import string
  import random

  directory_context = string.replace("/", ".", directory)
  return directory + "/" + 
random.choice(context.directory_context.objectValues('Image'))

Some of you may recognize this as being almost an exact copy of the script on 
p151 of "The Zope Book", with the only difference being that I'm trying to 
pass a folder name as a parameter.

I'm pretty sure I need to do the directory_context step, but I can't get it to 
work either with or without it.  Where I'm really struggling is in getting my 
head around the context stuff.

Can anyone offer me some assistance?

Thanks in advance

Dave Mitchell