EXCLUSIVE

"Justified" producers are under fire from an artist who claims they ripped off one of his paintings for their promotional art ... and now, he's demanding they ditch the ads -- OR ELSE.

Attorney Keith Davidson fired off a cease and desist letter on behalf of artist Gabe Leonard ... accusing FX Networks -- which produces the hit show -- of copying his original piece "Double Fist" in its ads for the new season.

The letter cites several similarities -- rugged U.S. Marshals, pistols drawn, hats tipped, color schemes -- and concludes its a clear case of copyright infringement.

FYI -- "Double Fist" had been posted on Leonard's website 3 year prior to the unveiling of FX's promotional art. FYI -- "Double Fist" had been posted on Leonard's website 3 year prior to the unveiling of FX's promotional art.