[Zope] Random Images: How to defeat the cache

[Edward Pollard]

I'm trying to make a quick and dirty random image product.

It is a zClass based off of OFS::Folder.

The index_html method is as follows:
context.REQUEST.RESPONSE.setHeader( 'Cache-Control' ,'no-cache' )
context.REQUEST.RESPONSE.setHeader( 'Pragma' ,'no-cache' )

image_set = []
images = context.objectValues('Image')

for each in images:
    image_set.append(each)

return_image = image_set[random.choice(range(0,len(image_set)))]
return return_image.index_html(context.REQUEST,context.REQUEST.RESPONSE)


This works fine as long as one shift-reloads the image. Otherwise it 
does not change the image. Obviously my attempts to defeat the browser 
cache are not succeeding.

Can anyone tell me why?