the flash image isn't extracted and I have an 8GB sd card in my phone
ok then there is your issue. you need to extract both files and move them to the sd card.
from the guide:
4.we need to extract and then move the flash_image and the mtd-eng files on to the sd card. make sure that you do not put the extracted files in any folders. they need to be on the root of the card (very top level)