What program are you using to create the slideshow?

I'll assume it's iPhoto. Check the version, I tried to recreate the problem using iPhoto '09 and wasn't able to. I created an mpeg-4 movie which I saved to my desktop and previewed in quicktime 7. It worked no problem.

It's the long route but you could rotate and export the images then re-import them and then create the slideshow. Just an idea.

Good luck with it.