I transferred some pictures from my PC to my iPhone 4G. I am unable to delete these pictures that were sent to my iPhone.
If you are using an iPhone 3GS, then you may still be on iOS 4. At this OS level, the ability to delete synced photos was not available, and you would only remove photos that you took with your camera in the camera roll. The only way to remove them, is to remove it from your sync list, and sync again.