Although external identification requires sharp eyes to spot it down at the bottom of the back of the phone, the iPhone 3G is model number A1241 and the iPhone 3GS is A1303. The China-only iPhone 3G and iPhone 3GS models that do not have Wi-Fi are model number A1324 and A1325, respectively.
I cant make out the model number in your photo…
https://everymac.com/systems/apple/iphone/iphone-faq/differences-between-iphone-3g-iphone-3gs.html