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, both programs provided with OS X. 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.
These two photos are used to demonstrate the problem and to offer a possible solution. See how here.