Create a new folder named "(Whatever You Want)".theme>>make a folder within it called "Bundles">>and another one within that one and name it "com.apple.springboard">> then put these two carrier logos in there. drag into winterboard themes using iPhoneTunnel or iPhonebrowser (google them). go to WinterBoard and check off the theme (what ever you named it) and there you should have it.
this one should be named...
Default_CARRIER_ATT.png
this one should be named...
FSO_CARRIER_ATT.png
(there's an image there you can't see it cuz it's white) right click and save