My wife's iPhone 3GS had a problem. The battery swelled and split the back! So I decided to save the 3GS and replace the back and the battery.
I always say, "If it's worth doing right, it's worth doing it 3 times"! After downloading a battery replacement video from the internet, I ordered a battery and new case and dove right in. It is amazing how Apple can make millions of these units. I have never seen such small screws in my life! However with a lot of patients the repair went very well. Everything works!!!
Be sure the workspace is clean and well lite. Try and keep the screws separated and remember where which ones go where as some are long and some are short. Take your time and follow the directions. After you've done it a few times, it's a piece of cake!
Sorry no photos.