Problem to make a DEBIAN LIVE USB PENDRIVE bootable? not bootable :(

[yellow protoss <yellowprotoss@gmail.com>]

Hello,

I download the several HDD-USB img images for making a pendrive. 
I tried on 2 pc and the result is similar : 
   they arent bootable.

I have done as such:
http://cdimage.debian.org/cdimage/release/current-live/i386/usb-hdd/debian-live-503-i386-gnome-desktop.img

and this one too I tried:
http://cdimage.debian.org/cdimage/release/current-live/i386/usb-hdd/debian-live-503-i386-standard.img

then 
dd if=debian-live-503-i386-gnome-desktop.img  of=/dev/sda1

So up to now, nothnig complicated, all is fine and well done. The img is copying to the right /dev/sda1
ok
done without error

So the image hdd usb pendrive is on the pendrive.

I reboot the machine, and select the usb pendrive at boot , and it says nothing
black screen, nothing is booted. 

Maybe shall I use syslinux to put a boot?

I am bit surprised because to install linux installer on a pendrive is possible

http://blogs.koolwal.net/2009/02/25/installing-linux-on-usb-part-7-install-debian-linux-from-usb-drive/

but it seems more complicated and not working with debian live. 

Any help is naturally very welcome !!

Kind regards
Y.