Other desktop environment's check /usr/share/icons for icons. However, if there's an icon there for a particular .desktop entry in /usr/share/icons, then the menu doesn't make use of it. I'm fairly sure that this is part of Freedesktop.org's Icon Theme Specification (http://standards.freedesktop.org/icon-theme-spec/latest/). Reproducible: Always Steps to Reproduce: 1. Put an icon in /usr/share/icons 2. Make a .desktop refer to the icon 3. Open desktop menu, and not see the icon.
Assuming you mean that the file directly in /usr/share/icons (e.g. /usr/share/icons/myapp.png), this is incorrect. /usr/share/icons is for themable icons only. Random non-themed icons should be stored in /usr/share/pixmaps. Generic app icons that are intended to be themed should be placed in the appropriate directory under /usr/share/icons/hicolor/.