Pictures taken on an iPhone go into the "Camera Roll" folder and can be easily deleted. Simply tap on the trash-can icon at the bottom of the picture and watch it disappear. However, you can't delete picture folders in this manner. Just like you had to sync your iPhone to add these pictures, you will need to re-sync your iPhone to remove them.
Launch iTunes. You must be using the same account you originally used to sync the picture folders.
Connect your iPhone to the computer with your USB cable.
Click on "iPhone" under "Devices" in the menu on the left side of the page.
Click on the "Photos" tab at the top of the page.
Un-check the boxes beside any folders you wish to delete.
Click on "Apply" at the bottom-right of the page. This will sync your iPhone.
If you no longer use or have access to the iTunes account you originally used to sync your iPhone, simply sync your device to the photo preferences in your new iTunes account. After you click "Apply" you will see a pop-up box asking you wish to delete the synced photos currently on the iPhone. Deleting synced photos will not affect the photos in the "Camera Roll".