Landscape and portrait pictures tend not to mix too happily in a slideshow: make the viewing area the right size and orientation for one and the other gets cropped or otherwise mistreated in unmentionable ways.
This can easily be avoided in Sandvox, but some preparative work is required.
For most of my graphic requirements, I use the tools provided in iPhoto and Preview. A shareware program called Graphic Converter adds those graphic requirements that these applications do not possess (both iPhoto and Preview have some amazing possibilities), including the ability to adjust the size of the canvas. What is more, the size can be adjusted in any and all directions, so if you want an image nicely centred, no problem, nor is it a problem to place an image in one corner, for example, should you so desire.
Here are the two images I have used in the slideshows here:
You can see what the problem is: the landscape oriented photo (the one above) is wider and not as tall as the portrait oriented photo (the one to the right). Neither can be depicted at full size in the other's space. Furthermore, Sandvox does not clear the image area when a smaller image is placed into it, so that remnants of the previous image remain visible.
For the simple "solution" slideshow here, I adjusted the size of the canvas of the landscape photo to be the same as that of the portrait photo, at the same time automatically placing the photo in the centre of the canvas (that part got rid of the problem of fitting one photo into another's space); I then filled the excess canvas (that above and below the image) with a colour exactly matching the colour used in the Sandvox design being used (that part got rid of the image remnants, though you could use any colour, of course).