background image auto size

Hi! I've already seen the question about the size for different screens
You talked about CSS3 and you posted some links where we could see how to do it.
But i Couldn't do it! Actually I know nothing about CSS! Everything I did on my page was using cargo support!!
Can you help me generating the right code?
This is what i have now and it's not working:


body {

background-size: auto;
background-image: url(https://dl.dropbox.com/u/5341861/ENTRADA%20PRINCIPAL.jpg);
color: #ffcc00;
font-family: Helvetica, Arial, sans-serif;
font-size: 12px;
line-height: 1.5;


}