10.6.3 wallpaper issue

If you mean that you see the purple aurora on the log-in screen, that's normal.

If you mean that you have a rotating desktop, and that it always reverts to one after a reboot, that's a lasting error that may or may not have been fixed with 10.6.3.

Try trashing ~/Library/Desktop.plist. Also trash Finder.plist, and (make a backup first) systempreferences.plist.

Go to your home folder. Find a folder called Library, and in there, a folder called Preferences. The .plists will be in there.

Did you restart after removing those files?

I have had this problem since 10.6.3 on my main screen only. However, I have now discovered that the problem occurs only when I set a 'change picture' image from an event in my iPhoto library. If I choose a single picture I don't seem to get the problem.

My iPhoto library is on an external drive and I note that the image is set on the main screen early in the startup/login, but much later on the second screen. So I wonder if the external drive is not quite ready when osx tries to get the new image for the first screen - since it doesn't know what image to get. If the image is fixed I assume it is cached and the os doesn't need to look for the image? By the time it needs the image for the second screen the external drive is ready?

Did you try any of the other solutions on the thread?

I don't think anyone's suggested this, but try re-installing the 10.6.3 combo update.