Hm. That's a first for me. I know of a few tweaks that can result in imagery problems like that but that's only on jailbroken or developer cracked iDevices. Well since you're looking to update it to iOS 6 and I was about to suggest that as well, let's jump to it. I (and others) recommend putting the iPhone into DFU mode first before restoring to make sure that we've covered all the possible loose ends to ensure a successful restore. Download the iOS 6.0.1 IPSW for your iPhone (make sure you get the type correct as well), open up iTunes or RedSn0w, restore and select the IPSW you downloaded and wait for the magic to happen.