icons missing or x-'ed out

[David Williamson]

For some reason, all of a sudden my desktop has some icon problems.  The
show-desktop icon, the trashcan icon - both actually on the panel - are
now both the 'blank page with red x' icon that usually means the image
you're looking for isn't there anymore.  This is the case with some
icons in my menus too.  It may have happened when I installed some
themes earlier today, but going back to the default theme makes no
difference.

I'm not even sure where to start on this one; any suggestions?

david williamson