Every so often, my system tray icons lose their pictures, usually because the server that they point to was offline for a while. Then I have to go hunting for the solution, even the one I posted on SuperUser. Here it is for future reference.
Open Windows Explorer, go to
%userprofile%\AppData\Local, delete IconCache.db.
Open Task Manager, go to Processes tab, highlight explorer.exe, and click End Process. Your desktop will "disappear."
Still in Task Manger, select File > New Task (Run…), type explorer.exe, and click OK. Your desktop should reappear, with the previously missing icons.