> I'm not familiar with scanning rules in httrack
Filters are generally used to limit what you get. Filters are also used to
override httrack's normal scanning such as scan down only and stay on one
site.

> I want to download from a phpBB forum (ex
> www.phpBBforum.com)
If you want everything, no filters. If you want just one subforum or only one
thread then you find the common part of the url and reject everything else: -*
+*12345*

> . all images
Without filters, options -> Links -> get non-html is sufficient.  With
filters, you'll have to override your filters -* +*12345* +*.jpg

> . excluding all pages, all files and all images
> outside www.phpBBforum.com
By default httrack will stay on a site. With non-html unchecked you'll get
nothing outside.