Originally Posted by enso
And can you describe how exactly do you modify it?
You mount the image file as a disk image. Then you read the raw ext3 disk with a ext2fs or other file system tool that can read and write linux disk images. Some such tools may be able to explore a disk image file directly without mounting it. Or it may be easier to loop-mount it using linux, or a linux VM in Windows.
Asking a question that contains the word "exactly" is considered rude. You need to learn some stuff on your own, or you should not be messing with this stuff... We can only provide GENERAL guidelines, not exact recipes for all special cases, especially when we do not know what problems you will have to solve along the way...