I use third party camera app called "Camera Plus Pro". This has great features like Tap to Focus, Exposure Slide and White Balance Lock. Since 3GS does not have a flash, these features help me a lot to get brighter pictures. Also, after you have taken a photo, you can apply Flash feature on your photo to make it look brighter.
Also, i apply lots of filter effects on my photo and directly upload the pics to facebook from the app itself.