There is a function called plogger_init_search in plogger-functions.php. Check out the MySQL Query...
$query .= "`path` LIKE '%".mysql_escape_string($term)."%' OR `description` LIKE '%".mysql_escape_string($term)."%' OR `comment` LIKE '%".mysql_escape_string($term)."%' OR `caption` LIKE '%".mysql_escape_string($term)."%' OR ";
That searches on file path (and filename), description, comment, and caption. You can remove the first line with the path to get rid of that functionality. So...
$query .= "`description` LIKE '%".mysql_escape_string($term)."%' OR `comment` LIKE '%".mysql_escape_string($term)."%' OR `caption` LIKE '%".mysql_escape_string($term)."%' OR ";