Ok, so everyone that's mentioning that you can't change the boot image, it's not true. Surprised I found it elsewhere first, but I will share...it's a simple process.
rainbow logo pic mirror :
http://www.mediafire.com/?24b4su2eemg
original link :
http://rapidshare.com/files/10460924...logo.img2.html
1) find your PwnageTool.app
2) right click and click "Show package contents"
3) navigate to Contents/Resources/*firmware version you're going to use*
4) rename the retro apple logo from "applelogo.img2" to "logo.img2"
5) overwrite logo.img2 with the retro apple logo file (backup the original file if you want to)
6) now create your custom firmware and make sure you check "Use DevTeam custom pictures"
Note: You can also add files to be included in the ipsw when it is built here
There you go. Now, getting OTHER pictures to work, that's beyond me. I just managed to get the rainbow apple logo in the right format. But to all who want it, there it is.
Thank me if that was useful at all?
