Deleting Photos from an iPhoto Library:
1 - from an Event or the Photos mode: select the photo(s) and use the Delete key to move the photos to the trash bin. Then empty the iPhoto Trash bin as follows:
2 - from an album, smart album, book, slideshow, card, etc.: select the photo(s) and use the key combination of Command+Option+Delete to move the photos to the trash bin. Then empty the trash bin as above.
NOTE: deleting a photo from an album, slideshow, book, etc., with only the Delete key only deletes that photo from that item. Deleting a photo from an Event deletes ALL occurances of that photo in the library.