This is maybe just the beginning but I thought I share it already because in the next couple of weeks I have no time to continue working on these scripts. I recently created a slideshow in the Video Sequence Editor (VSE) of Blender. The VSE has originally not been designed for creating slideshows (e.g. in contrast to the Windows Movie Maker) but with a few scripts, which restore the aspect ratio of images and which add pans and zooms to an image, it is pretty easy and straightforward to put a slideshow together. The scripts and a more detailed description can be found in this post at BlenderArtists.org.