iPhone 4 previous back up help
There are two ways to back up iPhones. One is through iTunes, and the other is through iCloud (seeing that you have an iPhone 4S). 

If you have enabled iCloud backup, then you can restore your phone by wiping it (Settings > General > Reset > Erase All Content and Settings) and tapping Restore from iCloud backup after your phone comes up again, then logging into your iCloud backup. Note that iCloud only saves the "latest" backup, and may not have the pictures if you actually (accidentally) deleted them from your phone.

If you have backed it up through iTunes, then plug in your phone, select "Restore" (I didn't walk through these steps myself just now, so they may not be click by click accurate), and select the backup that you would like to restore to your phone. I'm not sure if iTunes just saves the latest or if it will save multiple versions of your phone.