I have a 4g, so I don’t know if they are the same. But, it sounds like you have the same trouble I did when I first got mine. So, I’ll pretend they are the same and give you what I found.
You actually have 2 wallpapers.
The first is the one you are probably trying to set. This one is 960–800. Doesn’t sound right, since it’s wider than it is tall, right? Well, thats because your wallpaper is 5 screens wide. If you take your finger and scroll left or right, you get more apps. As you scroll, you will also see more of your wallpaper, as it scrolls across it too. You’ll only see one fifth of your wallpaper at any given time.
You also have a wallpaper at your lock screen. I don’t know the exact size, but the screen is, what, 480–800? I’m going to assume that the lock wallpaper is that size.
Now the real question is, is there a way to lock the wallpaper in place, so that when you are navigating/scrolling left and right to other screens, your wallpaper stays in place? That way, you could also use a 480–800 wallpaper for that also.
I don’t know the answer to that yet