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You need to learn how to keep backups safe before messing with the file system. Put them in a folder somewhere away from the folders you are using to swap images. I'm glad this forum isn't flooded with threads like these, it could easily be.

Why don't you just download the firmware file then decrypt it yourself to extract the files? The information for how to do this is readily available. I don't know that 4S keys have been posted yet. You can get iphone 4 5.0 firmware files decrypted.
 
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