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Adding to a full page. If you drag a photo from the Photo pane to a page that already contains photos in each photo frame, iPhoto changes the page type, adding an additional photo frame to accommodate the photo.
With some themes and page designs, iPhoto may add more than one photo frame. For example, in the Picture Book theme, the available page types jump from four photos per page to six. If you add a photo to a fully populated four-photo page, iPhoto switches to the six-photo page type. You can either add a sixth photo or ignore the empty frame. iPhoto will omit it when you order your book.
Moving and swapping. To move a photo to a different frame, drag it there. If that frame already contains a photo, iPhoto swaps the two photos.