Find out a solution:
In component -> com_jcomment -> tpl -> default -> tpl_comment.php changed:
<div class="comment-avatar">
...
</div>
to
<div style="border: 10px white solid;" class="comment-avatar">
<?php
$avatar_query = mysql_query("SELECT * FROM `phpbb_users` WHERE username='$comment->author'");
while($avatar_rows = mysql_fetch_array($avatar_query))
{
$avatar = $avatar_rows['user_avatar'];
}
if ($avatar != "")
{
$avatar = ("<img alt=\"user avatar\" src=\"../forum/download/file.php?avatar=$avatar\" />");
}
else
{
$avatar = "<img alt=\"user avatar\" src=\"../forum/styles/bridgesilver/theme/images/no_avatar.gif\" />";
}
echo $avatar;
?>
</div>
But still can't find out how to correctly link to profile preview of phpbb3 with those hashes...