Well As long as I know how to keep it from happening Im happy
So if you cant reproduce it then no worries Just keep details in back of mind in case someone else has same issue. Then you could at least ask if they have an image stretched or centered and or off.
If I can offer any other help let me knw

:dex:,

doesn't help. :x
I've tested the two scenarios. No problem.

all running 1024X768 60htz
I have Zune style on all desktops
and just to see if I could alter things I modified my desktop background images to be different on each desktop (new picture for each) then what they were before and now it is Desktop 2 that this issue is happening on
:!: :!: :!:
I noticed that the image on desktop 2 was centered while the rest were streched
I changed that and all are working correctly
Now thinking back to the original problem when it was desktop 4 I had no background image selected and was just using the color as a background.
So it would appear that it is the image being off on the one desktop or just centered and not streched.
Does that help

:dex:,

do you have a hint on what is different with the fourth wallpaper? File format?

Taskbar Shuffle
partial Zune theme such as the start button
Nothing else visual wise

:dex:,

could you provide a list of all your apps running (viz. apps that add visual effects to your desktop or something like that)?

I have 4 desktops
each has a different background image
3 work fine but the the 4th desktop image is usually the last desktops image that I was on prior to switching to the 4th desktop.

Also If I drag a screen on that desktop the screen leaves a trail that eventually refreshes.
I have turn desktop acceleration on and off but no change

Finally I left the 4th desktop on (with the wrong background image) and after about 10 minutes it switched to the correct image.

XP SP3 with all the latest updates
running version 1.5.0,build 1003