Judge Roger Chapple, at Inner London Crown Court, told Umaniec: "Your actions on the 7th of October of this year were entirely deliberate, planned and intentional."
He said it was "abundantly clear" that Umaniec was "plainly an intelligent man" and told the court he had described Rothko as a "great painter" in a letter he had written to him.
The judge also said the incident had led to galleries reviewing security arrangements at a cost to themselves and the taxpayer.
"The effects of such security reviews is to distance the public from the works of art they come to enjoy," he said.
More top news
A second man has been found guilty for the manslaughter of delivery driver Ali Qasemi who was killed for £52 worth of pizza.
A 32-year-old man has been arrested at Birmingham Airport on suspicion of funding terrorism.
The MP for Kettering has pleaded with Lindsay Lohan to honour her Twitter pledge to switch on the town's Christmas lights.