如何在页面上向下滚动时调整图像大小？

I am creating a nav bar for a website, however I can't seem to get this to work. I want the logo to shrink as you scroll down on the site. I've tried the webkit animation stuff, and a few javascript/jQuery functions but they don't want to cooperate. This is the current function i've played with and it doesn't seem to like it. How do I fix it?I am creating a nav bar for a website, however