It is relatively straight forward to resolve this. First, I would suggest a factory reset as it sounds that this was not done. The reason for this is not just the previous owners data will definitely disappear, but also, it would clear out any crap left behind on the system partition he may have had installed and therefore make the phone overall feel speedier.
If you don't want to go that route, as you would have to safe your data (either external micro SD or your PC) and then transfer it back again afterwards, at least you need to delete any Google accounts associated with the previous owner. His/your Google account will automatically sync with the server whenever a change is detected. You will find the accounts in Settings > General > Accounts > Google and there you should find either the previous owners or yours or both. If he has left his then you need to delete it.
If you did find another account then go to the "Application manager", where you will find 4 tabs from left to right and you want to select the last on the right-hand side called "All"; here scroll down to "Gmail" and tap on it then tap "Clear cache" followed by "Clear data", this will delete anything in Gmail (accounts, settings, etc). Then go to "Google Account Manager" and do the same again, then "Google Play Services", "Google Play Store" and last "Google Services Framework"; NOW REBOOT your device. When rebooted it will ask you to sign into your Google account and it will now also sync any apps you have in your account, sing into your Gmail account and so on.
There should be no trace of the previous owners pictures. In addition, you can choose how Google saves any data (including pictures) by selecting in the Accounts section under Google which data is synced by ticking or unticking the relevant item. You can also choose to backup your photos, which take the most data to sync with Google servers for backup, over Wifi only if you got to the "Google+" app > Settings > General Settings >Auto-Backup and there you have a slider for turning the service on or off, as well as Backup Settings where you can choose a radial button for either "Wifi or mobile network" or "Over Wifi only"; obviously the latter will ensure you don't risk any backup happening whilst on a 3G data connection.
That's it and al said and done I'd recommend the factory reset.
