From The Document Foundation Wiki
If the icons that are not displayed correctly are still the same
the image file must have been corrupted during installation (or during your work). We suggest the following:
- You do a 'repair' from the LibreOffice Setup:
- Windows, run the Add/Remove installation program from Control Panel/Program ;
- Linux, run the setup script in the installation directory of LibreOffice.
- A more radical solution would be to uninstall and then install LibreOffice again. We recommend this option as a last resort.