Actually, Wallpapyrus IS displaying the image exactly the same as the original. I just noticed that even though I can make perfect gradients on my MAC they just don't show up as smooth on the s3, even in the gallery. So I guess it's 'problem solved'.
I hadn't thought of using a different launcher (yes, I'm on stock). Does Nova require root? I might look into it in the future, right now I'm too busy learning Tasker!