customize background image

i have the following login form, and i want to give some shadow (like the login form does) to its background image.

we have to use custom css, and yet, i don’t know what is the name of my background image login from.

how do i resolve this ? can i make it without changing the style.css or someother kind ? so i use the plugin and the css inside the plugin to customze my login page and form ?

thanks for your support i really love this plugin, i really do

http://wordpress.org/extend/plugins/custom-login/