Hi Andria,
You cannot use their web button code because you cannot add extra form elements to the page in ASP.NET WebForms. I use the email style link which does not require a form element, if you view the source of the page on this site you can easily see the syntax. You would use your own hosted_button_id
Note that I'm adding the button image from a CSS sprite as a background image on the link, but you could just as easily use an <img tag inside the link.
Hope that helps,
Joe