Why doesn't my wallpaper scroll?

At first, my question was why I had to crop my wallpaper wider than it seemed to be, but I saw that that question was already asked and answered here: https://android.stackexchange.com/questions/39124/setting-wallpaper-wrong-orientation So now my question is, if wallpaper is cropped wide so that it can scroll when you switch between screens, how come mine doesn't move? I am using a picture I downloaded from the internet on my phone, and as far as I can tell it doesn't move at all no matter what I do. If it matters, my phone is a Motorola Atrix HD.