It's no "Thomas Crown Affair" -- but The Situation has been accused of jacking a painting of himself via the Internet ... and the artist behind the image is ballistic.
The artist -- Celeste Gillis -- tells TMZ she contacted The Situation on Facebook last month to tell him she immortalized him on canvas -- and he could either have it for a price ... or credit her art website on his FB page.
Gillis claims The Sitch posted the moob-tacular image on his FB page without doing either (above) ... along with the adding-insult-to-injury caption, "THE MAN THE MYTH THE LEGEND."
Gillis tells TMZ she never asked the Sitch to take down the pic ... because she didn't think he would understand copyright laws.
No comment from his people.