Comment 3 for bug 746028

I explicitly tested this for karmic -> lucid -> maverick and it was working.

I tried again:
- my wallpaper for gdm is /usr/share/backgrounds/warty-final-ubuntu.png
- my own wallpaper is /usr/share/backgrounds/Aubergine_Sea_by_Wyatt_Kirby.jpg

Try open gimp, and change both:
- in my session, I immediately see the wallpaper changing while saving
- when selecting "switch user" which sent me back to gdm, I see my modified warty-final-ubuntu.png image.

So if the timestamps are correct (meaning, the new one > the old one), the cache will be refreshed. Are you doing any special tweaks in edubuntu?