Thanks.
Specifying the full path works (needs a reboot).
I also edited ~/darktable/build/share/darktable/darktable.desktop.in, for protection against rebuilds.
I keep having this same issue. But, when I edit the darktable.desktop file to include the icon path it doesnt work. I just dont have an icon. Im on ubuntu. what icon path did you use? I have several different icon png to choose from.