Could use a hand.

This is a hack that i would like to do but am just starting to read the php manual and don't know as to how to start I have a few Ideas of how to do it but putting the code right code in the right spots i am unsure of..

Ok what i want to do is post a little graphic below the avatar area in the showthread if the user is the top poster, kinda like an award hack pulled this out of the stats panel hack(I am not sure if it is ok to do this? it is just for our personal board nothing i would release as a hack)
I know in the showthread template i have to compare the below information and compare it to the posts userid, and if the user id's match then I would like to show the graphic.

PHP Code:

// Top poster
$topposters=$DB_site->query_first("SELECT username,posts,userid FROM user ORDER BY posts desc LIMIT 1"); 


Any help is appreciated and information on if i can do this to my board without getting in trouble for using someone elses idea or code..

Thanks In advance.