It should be "UI\Feedback\Resources\ResourceSupply.blp", but its not neccesary to overwrite it. Just go to "Advanced -> Game Interface..." and find "Icon - Food" and change that icon to the icon you wish to use.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.