After you create a saved slideshow (one that appears in the Source list), you can add more photos to it or delete photos from it at any time.
To add or remove photos:
To add more photos to a slideshow, do one of the following:
Select the photos you want to add, click the Add To button in the toolbar and choose Slideshow, and then select the slideshow you want to add the photo to (if you have more than one saved slideshow).
Drag the photos (from your iPhoto library, a CD, a DVD, or a hard disk) directly to the slideshow’s name in the Source list.
Note: If you add a photo from your hard disk, iPhoto automatically imports it into your photo library, if it’s not there already.
To delete a photo from the slideshow, select the slideshow in the Source list, select the photo in the photo browser at the top of the window, and then press Delete.