Quote from retaildaytrader:
Threatening anyone with violence is generally illegal, but I would say most threats made to ordinary people over the internet are not investigated. However, all threats against the President of the United States are investigated by the Secret Service. The Secret Service investigates several thousand of these cases each year and it is a felony to make such threats meaning you might spend over a year in a federal prison.
In addition to federal laws, there are also laws in every state which makes it a felony crime to threaten public officials and employees of the government. For example, threatening a meter maid is a felony crime.
Google the many cases which involve internet threats against public officials and the President. You will soon discover that its a really bad idea to make violent threats and depictions over the internet.