And my mother forwards me pictures from others, and she is on my do 
not block list.  I certainly don't want to have to manually display 
those images, since I've already stated my desire to view all images 
in her messages.  Or else, what's the purpose of adding e-mails to a 
do not block list if you don't honor it?

In your situation, it sounds like you simply are not a good candidate 
for this preference.  Or don't add people to your do not block list 
that might send you these kind of messages.

A user forward me a email because he was unsure this is spam with a 
bad attachment.
Unfortunately IMP show me direct the images in the original mail which 
contain a image with a special url, you now to look the recipient is a 
human person.

Why?

If $_prefs['image_replacement']['value'] = 1; is set, forwarded mails 
should not show images even if the sender is known.