you are close - just put the background css code to the body style:
body {
background: #243C5D url(http://www.longislandmartialarts.org/wsblog/wp-content/themes/moosadotheme/bground.jpg) left top repeat-x;
border: 2px solid #243C5D;
border-bottom: 1px solid #243C5D;
border-top: 3px solid #243C5D;
color: #243C5D;
font-family: 'Lucida Grande', 'Lucida Sans Unicode', Verdana, sans-serif;
margin: 0;
padding: 10;
padding-top: 10px
padding-bottom: 10px
padding-right: 10px
padding-left: 10px
}
and while you are editing style.css, you might as well clear up a bit and remove the useless #header styles including a stray line just before that, i.e. remove exactly all this:
background: #fff;
#header {
background: #fff;
border-bottom: 3px double #fff;
border-left: 1px solid #fff;
border-right: 1px solid fff;
border-top: 1px solid #fff;
font: italic normal 230% 'Times New Roman', Times, serif;
letter-spacing: 0.2em;
margin: 0;
}
#header a {
color: #fff;
text-decoration: none;
}
all beginnings seem difficult - but there is no way round learnig html and css, as they are the building blocks of anything to do with the web - php, js and all else can come later.
there a decent resources out there, w3school.com for instance, to read up on new topics, anytime you run into a challenge.
also, the default theme that comes with the wordpress install, is a good source to see 'how do they do it'.