Where are the 'stock wallpapers' stored on the htc Desire S?
OK, what I ended up doing was using the ASTRO file manager app to access /system/customize/resource on my phone, which seems to have the wallpaper graphics (amongs a load of other stuff).  I copied the wallpaper I wanted to customize to an easy-to-reach place on the SD card (Edit | Copy, then Paste, in ASTRO), and copied it to my PC.  Customized the wallpaper, and then copied it to the sdcard's DCIM directory via a USB cable (you can also use Bluetooth for this).  I set it as my phone's background and it worked!  Yay!

Kudos to the people at xda-developers.

For those interested, I just wanted to have my phone's make/model display as part of the background.  Here's my new background wallpaper: