It's actually more involved than any of these posts have mentioned. I just found out the hard way to completely delete photos from the iPhone.
Step 1: Back-up your photos to a computer. Make sure that you have any pictures you want to save already downloaded to iPhoto or some other place. I'm not a huge iPhoto fan, but it was the easiest place to put the pictures from photostream and then move them elsewhere on my computer.
Step 2: Turn off photostream. This deletes any pictures from the photostream on the phone or tablet.
Step 3: Plug the iPhone/iPad into any Mac computer and open the Image Capture app. Now select your device and you see that most of the pictures are in there twice. Now select BOTH COPIES of any that you want to delete and press the delete button on the bottom of the window (it's the red circle with a slash through it). DO NOT SELECT ONE COPY AND LEAVE THE OTHER THINKING THAT IT WILL STAY ON THE DEVICE...ask me why I know.
Step 4: Open iTunes and be pleasantly surprised that the 40000 gigs of space spent on photos has dropped to near zero.
Step 5: Like this post to help bump it to the top and help people clear wasted data from their iPhone/iPad.