This happens when the home screen displays icons for .desktop files for which the icons do not exist. The home screen loads .desktop files from /usr/share/applications/*.desktop and /home/user/.local/share/applications/*.desktop and stores the order of the icons in /home/user/.config/meegotouchhome-nokia/launcherbuttons.data. The desktop files in /usr/share/applications are (or should be) provided by the installation packages of each application, so reinstalling the applications should bring their icons back. Removing launcherbuttons.data should get rid of any stale entries in the icon order configuration. Removing .desktop files from the user's local applications directory will get rid of icons not provided by application packages.