CSS background images

Well, today, i started off looking for webhosts that supported PHP and MySQL. I found one
http://www.000webhost.com/
I felt like trying MySQL but....it looked really awkward and scary...jQuery looked really awkward and scary too. So i went back to the basics. CSS and HTML. So, i thought i'd give myself like a mini project to do. I decided that my mini project was going to be making a website for my notes. So...i wanted a header with a different background...but i forgot a lot of things. I found out how to embed my css into my html file here
http://www.w3schools.com/html/html_css.asp
<link rel="stylesheet" type="text/css" href="mystyle.css" />

and then i had to check how to put my image in.
http://www.w3schools.com/css/css_background.asp
body
{
background-image:url('img_tree.png');
}

...but...turns out it was too small! I then found out that there was a value called an "em" which changes the size of things in css.

I then took a look at this website http://www.google.com.hk/url?sa=t&source=web&cd=2&ved=0CCEQFjAB&url=http%3A%2F%2Fcss-tricks.com%2F766-how-to-resizeable-background-image%2F&ei=bOQTTsSnJeGhmQW-4dGbDg&usg=AFQjCNFxtMMTYzAwnOniWXxcDN8eGU9BMw which was mainly about Jquery...i completely did not understand. So i gave up and messed around by myself...and i got this...

Strangely enough theres a white space in between my wannabe header and my window. I need to fix that...