I don't understand how to use this.
What are the part I have to edit?

i'm not sure how to explain it any simpler than stated in the first post, if you have a specific question on what bits confusing you, then let me know

I wanna add pictures for more then one forums.
And still I don't understand what I have to edit of the code you posted. Can u mark it in red please?

it's literally copy and paste the whole "forumhome_forumbit_level2_post" template into a text editor, even word would do, find <if condition="$show['forumdescription']"><div class="smallfont">$forum[description]</div></if> and then paste <if condition="$forum[parentid]==XX OR $forum[parentid]==XX"><img src="$stylevars[imgdir_misc]/[forumhome]/$forum[forumid].gif" alt="$forum[title]" border="0" vspace="2 px" hspace="2 px" align="left"/></if> right below it

if you give me the link to your forums and tell me the forums, i'll even post the code on how to add it to those specific forums.

<a href="http://www.interfans.org/forum/" target="_blank">http://www.interfans.org/forum/</a>

I'd like to have an image for each subforums of the category FC INTERNAZIONALE

this is the code you'll need to add to that to the template:

and this will add the images I want to all the forums of that category?
The images are different in every forum

yup

I still don't understand how can this work :(
Where do I have to put the images, in what folder?

images go in the directory

images/misc/forumhome/

and you name the images after the forumid

i would say install the mod and then work backwards, you can right click on the red x and get the location and image name for each image from there.