up

Just attach images to posts, there's a button on post edit screen

Strange, it works on http://demo3.b2evolution.net/stable/blog2.php
but not on my blog. Did thumbnail depend on the skin ?

After few more test it works !!!!!

Where can I define the default size of the thumbnail ?

Is there anyway to convert existing post to use thumbnail instead of real images ?

Edit sizes in /conf/_advanced.php

Is there anyway to convert existing post to use thumbnail instead of real images ?

You will need to edit them manually

ok to edit the posts (even if it's not possible, there are more than 2000 posts).
Is the thumbnail generated when uploading the image or when linking it in the post ?

It's generating when you view linked image the first time.