Ok, so you know the wallpaper is a single image that covers all 7 screens, what you are asking is how you make the bit of the image on screen 7 the "home" screen bit?
You can change this by choosing menu - wallpaper then choose your image, then move the green rectangle across the image until the bit you want as the "home" image is in the middle, you might need to stretch or shrink the green rectangle to do this, but that's the basic idea.
Hope that helps!