[Seaside] Paths to pictures in CSS

[mimik at centrum.cz]

Hi, i am newbie with Seaside. I am trying to display a background image on a web page, which is declared in CSS file like that:

    body { background: url('back.png'); }

but I don't know where do I have to put the back.png file, in which directory. I am testing in on my localhost, so the URL path is http://localhost:9090/seaside/test. I think I must tell Seaside that this URL responds to some directory on my disk, some sort of Web root like in Apache, but I can't find it. Can you help me?