Option 1
Save the file: http://en.wikipedia.org/wiki/File:LibreOffice_3.3.1_Calc_Icon.png
Later edit your ~/.local/share/applications/libreoffice-calc.desktop
.
Find line with:
Icon=libreoffice-calc
And change it to your save file into:
Icon=*path-you-have-saved-it*
Option 2
Locate file icon in your system using search option. In my case it is like:
/usr/share/icons/Humanity/mimes/256/libreoffice-oasis-spreadsheet.svg
so your line should look like:
Icon=/usr/share/icons/Humanity/mimes/256/libreoffice-oasis-spreadsheet.svg