Search for the Image Capture application in Finder.
Should be very clear how to do it after this.
But just in case:
Icon of iphone will be on the left under Devices and your pictures should automatically pop up and then at the bottom it says Import to: and you select other and then select your external harddrive.
I found this answer in a different discussion forum and this is THE BEST and FASTEST way to do it so I wanted to share with you. Thanks to TownieBrownie for sharing this solution in the first place.