Wow, I'm not sure what it is but I'm having trouble with 960 x 800 pics as well. I simply CANNOT get a picture to "scroll" between the seven homescreens. Like some others that have posted here, my phone (HTC One X S) makes me crop the picture before setting it as wallpaper....there is no way around this. So I have to crop it and no matter what size I crop it to it just becomes a static picture - it never moves when I scroll to the next homescreen.
This is really frustrating. How are others getting it to work? I literally have dozens of 960 x 800 pics that I've tried. I really want it to bump over when I scroll to the next screen.

If anyone has solved this for the HTC One X PLEASE let me know. I would really appreciate it.