Default convert nk.nbf to .img?

Hello all.
I'm having difficulty in getting a new rom on my wizard. Something f**ked up with the last update and i cant seem to get it going. I need to update using the SD card and the problem is the OSimagetool doesn't recognise the nk.nbf file to put on the sd card.
Do i need to convert it to .img so that it will boot??
Thanks