The lines go into the very same file db_field_definitions.php Look at the bottom of the file for these lines:
// Example for additional image fields:
//$additional_image_fields['image_photographer'] = array($lang['image_photographer'], "text", 1);
// Example for additional user fields
//$additional_user_fields['user_adress'] = array($lang['user_adress'], "text", 1);
These are examples and are "commented out". The two forward slash characters that start the lines tell php to not process those lines. So if you want a location field, your line would look something like this:
$additional_image_fields['location'] = array($lang['location'], "text", 1);