The 960x800 is actually appropriate for larger screens. Based on my research, the wallpaper should be sized to 640x480, as the 480 corresponds to the correct height for the Ascend's screen, which is 320x480 resolution.
FYI, I tried cropping some images down to 320x480 thinking they would be an exact match, but that failed miserably, as the phone blew up the image and it looked like crap. Stick with 640x480 for the Ascend, and your image will come out centered, at least it does on LauncherPro. If you use wallpaper scrolling, you will have the entire image on screen at some point, while you will get the center part of the image if you disable wallpaper scrolling.
This may not apply to the stock Canvas launcher, but I don't know for sure, as I haven't tried it that way.