This is done using the "Watermark" template.
You want to click on background click on the background image box. A pop up window will come up. You want to choose upload image. Then browse and find where you saved the background. Click on it. It should show up on your blog, click the done button on the pop up window. DO NOT click out of background yet...You still have one more step to do for the background is NOT centered.
You want to make sure you click on the center alignment and uncheck the scroll with page. Make sure you click on the BIG orange APPLY TO BLOG button at the top right of the designer. Now your background is set!! WAIT don't click out yet. You have one last item to change if you are using the watermark template.
Click on the advanced tab then the backgrounds. All the way to the right is footer backgrounds. Click on that small box arrow and set to transparent or type it in the long white box. NOW you are DONE!!!
Thank you for using my background!
Happy Blogging~!