I have used httrack before, but I never really understood the rules of
configuring a session.

I would like to download all image files that are in many, many threads of a
forum.

For a generic forum. What should my minimum project settings be to download
all image files only and no other files whatsoever. Many images are linked
from filehosting sites like imageshack and etc. I want nothing but images. png
gif bmp jpg and etc.

What is the best configuration for this?