Display Properties > Desktop > Background displays hundreds of files!

> Running SP2 on a new computer, I have for the first time in years used
> one of the built-in "My ...." folders, putting about 250,000 image
> files in hundreds of folders under My Pictures. All was well until
> today when I attempted to choose a desktop background.
>
> It appears that Desktop Background searches the My Pictures structure
> and comes up with hundreds, if not thousands, of files from which to
> choose some wallpaper. The dialog box appears to have gone out of
> business for a few minutes during this accumulation of unwanted file
> names.
>
> How do I customize Desktop Background to refer to the c:\windows\
> folder only?
>
> If that's not possible, how do I specify the file extensions to which
> it should pay attention (i.e., .bmp only)?
>
> I'm comfortable with editing the registry.
>
> --David