I highlight two principal methods in which to do this.

Method 1 – Clicking

iPhoto does not come with any external applications defined as a default, so when you right click on an image, you will see the Edit in external editor grayed out. (It’s not grayed out in my screen capture because I had already defined Adobe Photoshop CS3 as the external application when I wrote this post).

To enable edits in an external application, select Preferences from the iPhoto menu.

Navigate to the General tab, and drop down the Edit photo option box. Select application, and navigate to the application of your choice. In my example, I navigated to the Applications folder, then Adobe Photoshop CS3, and then I selected the Adobe Photoshop CS3 application.

Once you’ve done that, you will see the Edit in external editor option is now available whenever you right-click on a photo in iPhoto.

The Edit button (or double-clicking a photo) will now automatically launch Adobe Photoshop CS3.

If you want to change this behavior and have it call up iPhoto’s edit window instead, just go back to Preferences, and select Edit photo: In main window.

Although you can save an edited image directly back to the iPhoto library, I’d advise that the image you edit in Photoshop be saved to a folder that is separate from your iPhoto library, say the Desktop.

This is because iPhoto’s help says that non-destructive edits is not guaranteed when you edit an image outside of iPhoto, although I noticed that I was still able to right-click on an image that was edited in Photoshop and saved to iPhoto, and have that photo Revert to Original.

Alternatively, to be extra safe, you might want to consider first creating a duplicate image by selecting Duplicate after a right-click on the photo, and edit that duplicate image.

Method 2 – Drag and drop

M Thomas reminds us that the way of the Mac is about drag and drop. Drag a photo from iPhoto and drop it onto the Photoshop application icon in the Dock.