Blogger Backgrounds work well with minima template. This means you will need to delete your old background. Don't worry ~ you won't loose your side bar page elements!
Selecting Minima template:
Go to "Layout" > Select "Pick New Template" > Select "Minima" template > Save Template.
Next applying blogger background:
* Click "Layout'" > "Page Elements"
* Click "Add a Gadget"
* Find "HTML/Java Script" and click the "+" sign
* In the large empty box, paste the entire HTML code of the background you choose into that area.
HTML CODE Example below
2 hours ago