I would like to build a custom IPSW File for 3GS.
I just want to replace an PNG File on the iPhone OS partition.

Is there a simple way, to open the iPhone OS partition to replace that file and then to pack it back into a custom IPSW.

I don't want to have jailbreak on the phone.

Maybe I can Jailbreak before restore, but afterthat i would like to have phone with a replaced file and without jb.

Thx for any ideas