First off, you'll need to download
GIMP. It's like a free Photoshop for the Mac. Note that it needs X11 to run; just Spotlight X11 to see if you have it.
Now open the image you wish to use as the wallpaper in GIMP. In the window the image is in, there is a menubar. In it, click 'Image'. Then click 'Scale Image'.
In the new window, make sure that you have Pixels selected in the dropdown menu. Then in the top field, input 320. In the bottom field, input 480. Click OK or Scale.
Save your image and sync it to the iPhone.
- John