Slideshow on Wallpaper not showing photos right side up

I have 2,100 photos in IPhoto. I set up my Snow Leopard to show a slideshow as my Wallpaper. I went through my photos on IPhoto and used the Rotate function to make sure all my photos were right side up. However, when my Wallpaper is showing as the slideshow, some of the photos are still coming through not right side up. Any comments/help? New to Apple 2 months by the way!