Allot of people already now how to do it buth i will give a quick guide how exactly do it. It's not that hard and it works for Windows & MacOSX users ! Just follow the steps below and you have recovery image in no time.
1) Take your ipsw file and rename it to .zip extension
2) Navigate to Firmware/all_flash/all_flash.m68ap.production/
3) There you will find applelogo.img2 & recoverymode.img2, that is the Boot and recovery logo.
4) Now take your img2 files and just copy it (with good names) to the folder in the .zip file.
5) rename now the .zip extension back to .ipsw and your done !
6) Now flash the firmware and your new images are there.
PS: This is only when you whanna keep your cooked firmware buth whant to change the images, otherwise you have to cook it all over again.