I've spoken to a number of devs for multi monitor wallpapers. One of those is called John's Adventures Wallpaper Switcher.
In a thread, located
here, he noted that it's most likely Vista's fault that this is happening. Just so you know, from my research, this is a genuine VISTA bug. It's not any wallpaper program's fault. Bottom line:
Windows does not tile/stretch/crop wallpapers correctly IF the SECONDARY SCREEN IS TO THE LEFT OF THE PRIMARY (through display properties, monitor 2 is to the left of monitor 1). I ruled out nVidia drivers or any other 3rd party program, because it does this without any 3rd party software. Once you move the secondary screen ANYWHERE ELSE OTHER THAN THE LEFT SIDE, the wallpapers are rendered CORRECTLY, and you will get proper transparency through the main taskbar.
The current workarounds:
1. Enable Auto Hide for the main taskbar.
2. Make the left monitor (monitor #2) the primary, but move the main taskbar to the right monitor.
3. Disable transparency system wide (ignore the problem)
4. Use Windows wallpaper, and set the wallpaper position option to the first one on the left. For some reason, this is the ONLY setting that works properly in the entire scenario.