Making Custom Theme - Wallpaper Display Issue

Hello. Haven't been able to find an answer to my problem, so I'm posting it here. I have an iPhone 4 (CDMA) on 4.2.8 and I'm working on a custom theme. I've run into a problem with the way that the wallpaper is displayed. I've created my theme folder and have the Wallpaper.png file in the top-level directory for my theme. It does display the image when I activate the theme, but it "zooms in" and displays just the top-left portion of it. The image is 640x960, which I understand is the correct dimensions. All the other aspects of the theme (icons and dock) work fine.

Any help would be appreciated, I'm new to the process of creating iPhone themes.