Per inserire questo sfondo nel tuo blog vai in Blogger, clicca su Modello e poi trova questa porzione di codice:

<Variable name="body.background" description="Body Background" type="background" color="$(body.background.color)" default="$(color) url(//www.blogblog.com/1kt/watermark/body_background_birds.png) repeat scroll top left" value="#89b3c9 url(//themes.googleusercontent.com/image?id=1R2SvPiwXOrJBSCT0FujU9zHTzXtPNTVSq-0U2bZnGWq45BGCRfhp7tPktR9Y8Y7ecx1M) no-repeat fixed top center /* Credit: tjasam (http://www.istockphoto.com/googleimages.php?id=9401110&platform=blogger) */"/>

e sostituiscila con:

<Variable name="body.background" description="Body Background" type="background" color="$(body.background.color)" default="$(color) url(https://www.iolecal.it/blog/posts300/post327/silviaSfondo.jpg) repeat-y scroll center center" value="#89b3c9 url(https://www.iolecal.it/blog/posts300/post327/silviaSfondo.jpg) repeat-y scroll center center"/>

Infine salva le modifiche.