I was messing about trying to make the desktop background work well under Opie, and I got very confused about the size of the image. Then I realised what was happening.
If this is repeat information for you then move on, but I found it not to be immediately transparent.
1) The background is scaled to the width of the display as it is oriented when you set it in the launcher.
This means that if you set your 640x480 landscape image as the background while in portrait mode it will be scaled to 480 wide and tiled. When you switch to landscape orientation this setting sticks, so you'll get tiles of your image to the right and bottom of the screen
If your image is oriented to landscape then set the display to landscape before setting the image as your background.
2) Parts of a 640x480 image will always be hidden or tiled no matter what orientation you are looking at.
If you are looking at an image in landscape the top of the image starts just below the tab bar at the top of the screen and the bottom of the image is hidden by the menu bar at the bottom of the screen.
In portrait mode this bottom part of the image is revealed, a portion of the top of the image is tiled below it and roughly a quarter of the right hand side is hidden.
Well this is a lot to think about and all I wanted was an easy way to make my own desktop backgrounds. What's needed is a template!
So I made one.
This was done using OZ/Opie 184.108.40.206
Download this file to your Z with a 640x480 screen and set it as a background by editing a tab in the launcher. When you set it make sure you are in landscape mode.
You will see the rulers in the image on the top and left, several red guide lines, and some descriptions of how different areas of the image will be displayed. Once you see how things work you can open the file in your favorite image editor and layer stuff right over it.
This works great on my 3200 and makes it possible for me to begin designing desktops that work well and don't get skewed or scaled. You can also take advantage of the hiding and tiling to create layouts that work in both modes.
Please let me know if this works for you or not. In a while I can do one for the smaller screens. I have a 5500 as well.
Hope all this is useful despite the frivolity of it all.