1st of all I hope this is the right board for these, 2ndly I made them for myself, so i hope they are of use to other people as well
They get uploaded to the /System/Library/Carrier Bundles/ folder. In there there will be the folder for your carrier, I`m on O2-uk, so I have uploaded into /O2_UK.bundle/
The files that are being replaced are Default_CARRIER_O2-UK.png & FSO_CARRIER_O2-UK.png
obviously save your original files and make sure and name these files correctly for your carrier