Agents in Bush gun pic probe.
The picture, titled Patriot Act, by artist Al Brandtner, showed the image on a sheet of postage stamps.
Staff at a college in Chicago were stunned when two agents turned up to the opening night of the exhibition and demanded to speak to the curator.
A secret service spokesman said they acted after a call from a concerned local.
He added: "We need to ensure as best we can that this is nothing more than artwork with a political statement."
The college said: "We stand firmly for freedom of expression and academic freedom."
The picture stayed on display.
ON SHOW: Picture