“No official or employee shall make or participate in the making of any governmental decision with respect to any matter in which he has any financial interest distinguishable from that of the general public ...”
The city of Chicago has an ethics code — a quite extensive one, 50 pages long. It makes for interesting reading. Public officials are forbidden from using the city seal in photos on their personal Christmas cards, since they mustn’t include its weird symbolism — why is that naked baby on a clamshell? — in snapshots “not related to official City business.”