Android os source usage

Hi. I searched internet lots lots lots lots times but i still cannot
understand the
concept so im asking in here.

1.We need kernel image, bootloader, and filesystem to port Android2.3 os
into
my target board tiny6410, right? I achieved this by burning sd card and it
worked good.

2.DVD provides both kernel source code and OS source code. So, kernel
source code is needed for building an zImage file. OS source code also can
be built right? I did it and there are some files created such as
system.img, ramdisk.img, etc.. am i doing right?

3.So, This is my core question. Why do I need to build android OS source
file? How can we use built OS img files with tiny6410?

I mean, we only need kernel img, bootloader, and filesystem, right?
 How can we use OS built file?
How can we install android OS using OS built image files with my SD card?

nah, i dont have serial port on my labtop and usb-serial converter does not
work somehow.

Thanks in advance