Typically this can be done in you style.css file (or similar depending on template & blog). In wordpress you can modify your style.css by clicking on Design and then click on Theme Editor. Once in the theme editor you should see a list of pages and stylesheets on your right. For my theme, I can change the background by first selecting style.css from the list of stylesheets and modifying the Body tag as shown below.
Code:
body {
background-image: url(images/Tile001-150x150.gif);
} 'Tile001-150x150.gif' is the name of the image file I uploaded to my images directory of the theme I am using.