Sorry, but i am not here to code for you.
You have to read the material what has been provided for you.
You will need this one…
$w("Document").background.src = "place_here_the_url_link_of_your_background_pic";
Also read this one, not sure if you will need it…
Complete your code and make it work.
Always pay attention onto the → CONSOLE and it’s outputs!
Good luck!