How can I add a background image to an opt-in form on a WordPress site?

Do you know what plugin is being used to create an opt-in form with a background image on the right side of this image (hxxp://i.imgur.com/txGLdTi.png) and that scrolls the screen in the right sidebar of this site (hxxp://comodefiniroabdomen.net)? Regards, Daniel.