There~

I has decided to update in order to answer Akira's question.

First, you go to your dashboard.

Then, you click layout.

Once in layout, you'll see Edit HTML in the second row from the top left side of your screen.

Click it.

Next, press Ctrl F. You will see a box appear on the bottom of your screen[if you're using Firefox]. Type background-image in the box.

You will find something like this:

Paiseh. Very de blur..

There'll be a url there. Just change it to the url of the picture that you want.