I was cleaning out a bunch of kipple from my desktop directory and found the Lubuntu desktop screen wasn't updating as expected: the icons were mysteriously hanging around. Since PCManFM manages these icons, a simple solution is to shut them down and bring them back up:

pcmanfm --desktop-off
pcmanfm --desktop --profile lubuntu

Usefully, these same commands can be used to hide all of your icons before a presentation and then restore them afterwards.