In order to remove just the portion of the post that contains the image link info, you'd have to know the post_id. Not very simple if you have many posts and can't get into the thread to find out the id of the post. But if you know the thread_id and the forum_id, you can get the post_id by previewing this in custom pages:
<?php
$result = dbquery("SELECT post_id FROM ".$db_prefix."posts WHERE post_message LIKE '%[IMG]%' AND thread_id='1' AND forum_id='3'");
while ($row=dbarray($result)) {
echo "Post_id with [IMG] tag is ".$row['post_id']."<br />";
}
?>
This will ouput the post_id for you. Of course, you must change the numbers in red in the above code to the ones you need...then you browse the posts table, find the post, and edit the post_message to remove the offending part of the post. 
