Dude, Under the FileSet category in Customize, go to Wifi, then go down to the bottom of the list and look for a file called "iPhoneBars4WiFi," download the file and apply it in the Status Bar Images section.
1)It's fully functional to Represent the strength of the Wifi signal, but yeh, you need WiFi at that time to show that you have a celluar "signal."
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2)May I also recommened adding the MakeItMine application, instead of working so hard to make a custom Photo for a Carrier Image, you can just type and go.