You have the same phone but are you using the same app for pictures? I think that is the key. My photos are IMAG1234 with the stock app but I have probably 4 other camera apps which all use different naming systems. I'm not positive but I think Android default is IMAG. If somehow it turns out that you are both using the same app (the stock app) I can find no way to edit the default name. The only thing I can think is some hardware difference. While the phones are the same and were probably purchased at or near the same time, these device companies don't always use the same part for part make-up when building these devices. So you may have an S4721 chip and the other may have an S7782 chip which do almost exactly the same things but the 7782 chip only generates numerical outputs while the 4721 can also generate alpha bits. They just ran out of or started getting one or the other at a better price. If you are using the same app, just for S&Gs look at the serial numbers and see how close they are? (Never post your device's serial number online) I'd be curious what the difference is.