This can become extremely annoying especially if you are syncing your iPhone a lot and are not taking a lot of photos with it.

Open Image Capture

Goto the applications folder and open Image Capture application

Preferences

Click on the Image Capture Menu item and then open Preferences

Change Default Application

Click on the drop down menu and select “No Application” Then click the red x in the upper right hand corner of that dialoge box to close it and then quit Image Capture.

Quit iPhoto and you are all set.