I think I understand.. the actual buttons themselves?
In that case, you could either find the image src code and point it to yours, or simply overwrite the buttons with your new images:
/assets/default/images/open_ticket_btn.png
/assets/default/images/check_status_btn.png
Or.. I could be wrong, and it might actually be here:
/include/class.nav.php: (line 262)
function getNavLinks(){
global $cfg;
//Paths are based on the root dir.
if(!$this->navs){
$navs = array();
$user = $this->user;
$navs=array('desc'=>'Support Center Home','href'=>'index.php','title'=>'');
if($cfg && $cfg->isKnowledgebaseEnabled())
$navs=array('desc'=>'Knowledgebase','href'=>'kb/index.php','title'=>'');
$navs=array('desc'=>'Open New Ticket','href'=>'open.php','title'=>'');
if($user && $user->isValid()) {
if($cfg && $cfg->showRelatedTickets()) {
$navs=array('desc'=>sprintf('My Tickets (%d)',$user->getNumTickets()),
'href'=>'tickets.php',
'title'=>'Show all tickets');
} else {
$navs=array('desc'=>'View Ticket Thread',
'href'=>sprintf('tickets.php?id=%d',$user->getTicketID()),
'title'=>'View ticket status');
}
} else {
$navs=array('desc'=>'Check Ticket Status','href'=>'view.php','title'=>'');