[Zope] Image instead of ImageFile for product icon?

[Jean-François Ménard]

I use PIL to create on-the-fly icons for my python product.  So in my
product __init__, instead of ImageFile icons, I create Image icons.
Everything seems fine, but when I try to use it, I get a login dialog and,
on cancel:

Unauthorized
You are not authorized to access this resource.
Username and password are not correct.
Traceback (innermost last):
  File C:\PROGRA~1\Zope\lib\python\ZPublisher\Publish.py, line 150, in
publish_module
  File C:\PROGRA~1\Zope\lib\python\ZPublisher\Publish.py, line 114, in
publish
  File C:\PROGRA~1\Zope\lib\python\ZPublisher\Publish.py, line 89, in
publish
  File C:\PROGRA~1\Zope\lib\python\ZPublisher\BaseRequest.py, line 404, in
traverse
  File C:\PROGRA~1\Zope\lib\python\ZPublisher\HTTPResponse.py, line 511, in
unauthorized
Unauthorized: (see above)

It seems that it's a security problem...

Any clues?