[Plugin: WP Supersized] constraining proportions

Hello

I absolutely love wp-supersized.
Easy to install and use, and everything run smoothly…except that I cant get it to constrain the image proportions when resizing the browser.

Should be a simple task, no?

I have tried the different combinations in the controlpanel, but it doesn’t help.

I hope you can help with this:

test page

http://ww.wp.xz.cn/extend/plugins/wp-supersized/