Sería algo así:
Código PHP:
<?php
$link = mysql_connect("localhost", "root", "******");
mysql_select_db("forum",$link);
$sql = "select * from session where userid ='".mysql_escape_string($_POST['caja1'])."'";
$sql = mysql_query($sql) or die(mysql_error());
$row = mysql_num_rows($sql);
if($row == 1){
echo "<b>Conectado</b><img src=images/user_online.png></img>";
}
else
{
echo "<b>Desconectado</b><img src=images/user-offline.png></img>";
}
?>