The slideshow wide was really made to present a 'banner' type slideshow, so you would really need to make all of the images for this slideshow with the same proportions as 960 x 260 (or larger but still with a 48:13 w/h proportional ratio).
This might mean selecting areas of your images to crop to these proportions, or stretching/overlaying/gradient etc the images to fit the 48:13 ratio.
The alternative would require:
2. using a separate PowerGallery solution which would give you more flexibility (by being able to add custom dimensions to a global file), or
3. using a different gallery template that doesn't depend on proportional sizing.