A painting depicting a grinning President Reagan in blue jeans and cowboy hat astride a white horse is among the troublesome summit issues that advisers to Reagan and Soviet leader Mikhail S. Gorbachev will be trying to untangle.

Soviet authorities confiscated the painting by Louisiana artist George Rodrigue, which had hung briefly in a private Cajun food restaurant in Moscow that is serving gumbo and fried catfish to visiting U.S. government officials and journalists covering the summit.The Soviets also took away a painting showing Gorbachev in a yellow suit holding a red Soviet flag, with the Kremlin and a peasant family in the background. Officials said they will not return the art until they make sure Reagan is not insulted by his portrayal as a cowboy.

"The Soviets don't understand American art. They just don't know how to read this," said Rodrigue, a drawling, boot-clad cowboy born in Lafayette, La. "They don't appreciate that this is an individual's artistic interpretation, something not permitted here."

Rodrigue said that although the Soviets removed both paintings, it was the one of Reagan - not of Gorbachev - that concerned them.

The Reagan painting, copied for Republican Party posters, was presented to the president in New Orleans in 1986 and will be on exhibition until it is given to him permanently when he leaves office next January, the artist said.

The painting was displayed briefly in a restaurant that John D. Folse, a Cajun chef and owner of Lafitte's Landing Restaurant in Donaldsonville, La., was allowed to open here for the duration of the summit.