Thanks for your answer

But I'm not sure that it will correspond to my need, because :
- the phpBBforum links generate HTML files
- in this forum, main images are outside the forum. But by default, httrack
download all thumnails and jpg/pictures on my PC, and it takes a VERY long
time...
- I want to find (& download) in about 100K threads only few pages (about 50)
which contains somes png files (3 different files)

So I want to download html files :
- but only those which contain 3 different PNG pictures (pic1.png to
pic3.png)
- and I don't want to download images and thumbnails which are stored outside
the forum.

Is it possible ?