@ckh.
Simply reduce the height of the images, so open in an editor and reduce the height to the number of pixels you want. I think the slider will always fill to full width, and will crop if its taller than the maximum, but wont distort the image. So be best results (crisp and clear images) make the native image the exact pixel width of your slider and the height you want.
Possibly can achieve this via CSS, but I am a total noob to that side it! This method worked for me.