First, to make a new room you have to click here (make sure you're in author mode!):
Next, the new room should look like this:
Ok, so now you have your new room click on the 'My locker' button.
Now when you open in, it should look like this (But not with any backgrounds):
So now that you have your locker open click the 'Upload' button.
Now choose an image of your choice.It dosen't need to be full screen because worlize does that for you.
Now when you uploaded it click on it and you're done!
Thank you for reading this tutorial. Hope it helped!