Question:
What happens if the images are not the same height and width? I always have a mix of horizontal and vertical photos.
Answer:
Then all slide images will be forced to the same height/width as the base image (= the first slide image). And that's probably not what you would want to see in your slide show.  :)

But you can always "simulate" this same size by placing smaller images on a transparent background that has the same size as the largest image to be used in the slide show (= adding dummy 'white space' around smaller images).
You would do this in some graphics program like Fireworks or Photoshop of course, so the resulting GIFs or JPGs are all the same size.