First download the carrier icons you would like, macthemes has an excellent collection.
Originally Posted by carbaj12
1. SSH into your phone and navigate to /System/Library/Carrier Bundles/iPhone/ATT_US.bundle/
2. Find the files Default_CARRIER_ATT@2x.png, FSO_CARRIER_@2x.png, and TS_CARRIER_ATT@2x.png and back them up, so you can go back to the default images if you wish to.
3. Rename the new images to match the names from above and place in the same folder from step 1, now assuming you backed them up, you can go ahead overwrite them. Respring and you are done.
This might help also, when i first tried it, i was confused as to which file was named what, so fyi the default carrier will be either blue or black, the FSO and TS will be white or a light gray.
Hope this helps, i wrote this really quick, so let me if you have any questions.
Edit: i see you are on Sprint i wrote this assuming you were on ATT, im sure the steps are the same, just instead of seeing ATT you would see Sprint on the folder and filenames.