It should be as simple as this:

Icon=foo in /usr/share/applications/hildon/foo.desktop

a 26x26 in 26X26/hildon/foo.png
a 40x40 in 40x40/hildon/foo.png
a 64x64 in scalable/hildon/foo.png

no cheating, exact image sizes

exit all programs
gtk-update-icon-cache -f /usr/share/icons/hicolor
reboot

Works for me every time.