Deleting photos in iPhoto slideshow - need assistance asap!

I'm making a slideshow in iPhoto. I have tried to delete photos from the slideshow by first highlight the photo, then clicking on "Photo" in the menu and then clicking on "Delete from Album". I deleted several photos and it appeared as if they had been taken out of the slideshow. When I go back to view it the same photos that I thought I deleted again show up in the show. Huh? What am I doing wrong? I've got to finish this slideshow this weekend! Thanks in advance.