Firstly you need to make a picture that you want to be your background to be, don't make it to big as it will be repeated numerous times so it covers the whole page. Make it about 300x300 pixels.
Once you have finished making it, you need to upload it to a graphics host, or your site.
When you have done that, go to the page it is on and right click on it and go
to 'properties' and you will notice where it says 'address url'
It will have something like: http://www.yourwebsite.com/background.gif
You need that, so copy that url and paste it in a word document so you don't lose it. Then take this code:
<body background="YOUR IMG URL HERE">
And paste it just under it.
Now replace where it says: YOUR IMG URL HERE with the url you copied before. Make sure not to delete the: "'s
Now you should have something like:
Now go to the page(s) you want the background to show up, and paste the code! If you're using css, just paste the code into your css.
When you view your page your background should now show up =D
Did this tutorial help you? Please link me!