The "Usage ID" (key code) for the Keyboard PrintScreen key is 70 (=0x46).
You can send a raw Usage Code by adding 136 to it. So to send Printscreen, send the decimal byte value 206. Of course this only works for Usage Codes up to 119 (255-136) . Usage Codes higher than that can't be sent with the Keyboard library.
You can find the Usage Code table starting at page 53:
http://www.usb.org/developers/hidpage/Hut1_12v2.pdf