How do I make a vbulletin plugin ?

[nascimbeni]

How do I 'make' a plug in ?
Sorry I did not grasp how vbulletin works yet.
On the top of my forum ( next to the logo ) at http://my.lifeinitaly.com
I want to add a piece of php code that selects 4 / 5 random images. I hove the code and it works : you can see the images here
http://my.lifeinitaly.com/images.php ( the code is below ) - I need then to add something to the header template but what and how ?
I could not find any instruction on how to built a plugin

<?php
$host = 'localhost';
$dbUser = '';
$dbPass = '';
$db = 'photopost';
mysql_connect("$host", "$dbUser", "$dbPass") or die(mysql_error());
mysql_select_db("$db") or die(mysql_error());
$result = mysql_query("SELECT id,cat,bigimage,width,height FROM pp_photos WHERE storecat = '0' and width > height ORDER BY rand() LIMIT 6" ) or die(mysql_error());
$imgs1='';
while($row = mysql_fetch_array( $result )) {
$imgs1 = $imgs1 . "<a href=\"/photopost/showphoto.php?photo=" . $row['id'] . "\"><img width=105 height=75 src=\"/photopost/data/" . $row['cat'] . "/thumbs/" . $row['bigimage'] . "\" border=\"0\"></a>";

}
print $imgs1;
?>