Ok, this is kinda lame, but never the less it works. Just to give you idea how to do it yourself:

in showthread.php around line 450 you have something along the lines of:
$post[joindate]=vbdate($registereddateformat,$post[joindate]);

below add
include('/whatever/path/flags.php');

create file flags.php in /whatever/path and inside the file use
<?
$post[$field2] = ereg_replace("United Kingdom", "<img src=/path/to/your/flags/UK.jpg alt=\"United Kingdom\">", $post[field2]);
(...)
?>

To get eg:
Location: London [flag image] instead of just flag edit your postbit template and add $post[field1] after Location: ...