When you switch to edit view to edit a photo, it can open in one of two ways: in the iPhoto window or in another application.
To change how a photo opens in edit view:
Choose iPhoto > Preferences, and then click Advanced.
Choose an option from the Edit Photos pop-up menu:
In iPhoto: Opens the photo in iPhoto edit view.
In application: Opens the photo in another application you choose, such as Adobe Photoshop. When you choose this option, iPhoto opens your computer’s Application folder so you can select the application you want. This application then appears in the “Edit photos” pop-up menu. If you want to change this application to a different one, choose it in the “Edit photos” pop-up menu to open the Application folder.
Video clips you import into iPhoto always open in QuickTime Player.