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
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.