Login background what css

By nolox

My elgg site is private. So that way, users no logged in only view the login box in a white page. I wanna put an image background in that webpage. Thinks I tried with de css code "background-image:url('_graphics/login.jpg');"

-Put it in index.php---> Works great abut affect all site webpages and I only wanna change first page for non logged users

- Put it in differents styles of walledgarden.php---> It doesn't work anyway

- Put it in custom index css ---> It doesn't work, affect the page after login in.

Please help, what css stylesheet have I  to change?