iPhoto and Image Capture can import photos from the Camera Roll, not from the iPhone's Photo Library.
The iPhone cannot be used as an external drive - disk mode is not supported.
Photos in the iPhone's Photo Library include photos that were transferred from your computer via the iTunes sync/transfer process. Photos transferred from your computer to your iPhone are optimized for viewing on your iPhone via the iTunes sync process - the original resolution of these photos is reduced, which is why transferring these photos in the opposite direction is not supported. Photos transferred from your computer should remain on the computer's hard drive and be included with the computer's backup.
There are some 3rd party paid utilities that provide for transferring these photos in the opposite direction, but such utilities are not supported by Apple and the original resolution of these photos will be lost. Here is one such utility.