basically the icon in the Dock is just alias point to the application and you can just go to the applications folder and drag the iPhoto application to the Dock to place it there
And after doing what Larry said, don't forget to control-click (right mouse click) on the iPhoto dock icon and choose Options -> Keep in Dock, or the icon will disappear again when quitting iPhoto.
The latter is normal behaviour, since every application that's not nested in the Dock appears in it just while it's running.