Logo in header of invoice

I have problem whit logo on pdf invoice, i tried to do png smaller. But its same size, is it possible to add logo size.
I am not good in this:(

 $pdf->Image(FS_DIR_HTTP_ROOT . WS_DIR_IMAGES . 'logotype.png', 10, 5, null, 28);

/ Foppa

null=width, 28=height

See the reference for Image size:
http://www.fpdf.org/en/doc/image.htm

Thank, now I understand !