phpBB is attempting to use GD to scale windows bitmaps when GD only supports Wireless Bitmaps.

Only one change is needed, I think (I have not tested it).

includes/functions_posting.php:

// BMP, WBMP

case 6:

case 15:

     $new_type = ($format & IMG_WBMP) ? IMG_WBMP : false;

The first case needs removing. On a side note would it not be better to change the integers to the IMAGETYPE_* constants?