changing header image.

trying to change header image.

I know i need to change this url in the code

#root{
width:780px;
margin:0 auto;
border-top:0;
border-bottom:0;
background:url(images/body-back.jpg) 0 32px no-repeat;

I saved the image, resized the new one to be exact so i dont have to change margins and such. I got a domain name, had a link to word press site and thats where i’ve been doing the code/design. I have my image background.jpg on my desktop, I have no idea how to upload it, where to upload it or what to put as the new url.

on the wordpress site it says download word press version 2.7.1, i did that, see the folders, themes-images, but it doesnt have the theme im using, also none of these things seem to be associated with the wordpress site where im doing the code and posting anyways.

I confused.