[sockwater]

1. What is the exact error it gives you?
2. Are you certain $row[0] contains a valid userid? Use die($row[0]); to check.
3. Don't quote the integer value.
4. Are you sure $db is in scope?
5. What hook are you doing this on?

$userid = (int) $row[0];
die($userid); // use this for a sanity check...
$db->query_read("
        SELECT filename 
        FROM " . TABLE_PREFIX . "customprofilepic 
        WHERE userid = $userid
");