SlideShow Width

I have the slideshow in my header and am using almost the whole width of the header.

The pictures I am using are more than big enough – most 2000×4000. However, unless I stretch them, they do not fill the complete area:
http://dev.horticulture.wisc.edu.

I have played with the settings on both the actual slide show and the default settings and nothing seems to work.

If the pictures are large enough, what do I need to using in my settings to make them show up correctly. Stretching puts them out of proportion.

Thanks.

http://ww.wp.xz.cn/extend/plugins/slideshow-jquery-image-gallery/