HELP W/ BLOG BACKGROUND

Hello, friends! I am very very new to this website and require some help D: 

I have been messing around with my profile for awhile and I haven’t found a way to set my background as an image I want. How do I do this? I’m really no good with coding, and I unfortunately have not been able to change my template’s background correctly; as every time I attempt to add the link into where the previous image was (via replacement) it ends up just being a blank white background. Can anyone who is more familiar with coding these blog formats help me out? 

It would be much appreciated!