Proportionally Fitting Images In Slideshow 2?

I have set an overall size for the Slideshow 2! div in the CSS file as follows:

.slideshow {
height:250px;
margin: 0 auto;
width:420px;
}

I would then like all the images I use in the slideshow to fit within this area (i.e. 420 x 250). The images will all have a height of 250px, but different widths. For widths less than 420px I would like the image to be positioned centrally in the div along the horizontal axis as in the following image:

for images wider than 420px I would like them to be positioned centrally in the div along the vertical axis as in the following image: wonder if it's possible to achieve this with Slideshow 2! and if not, whether another script will help me achieve this?