It's not big, MySpace blog stylesheets are weird, they've decided to conveniently NOT accept the body tag, so here's what you can do:

Log into MySpace, and click on blog under the control panel. Scroll down to My Controls and select Customize My Blog. Scroll down until you see Background Settings and input the following:

See where it says paste url here? Just add in the URL from my code box or add the URL from photobucket (or wherever you uploaded the image).