EXCLUSIVE

Nobody photoshops fake nude photos of Kate Upton and gets away with it ... a lesson Sports Illustrated wants to teach a joke website called Celeb Jihad.

The phony photo pheatures a photoshopped pic from an SI shoot with the headline, "Kate Upton Naked Outtake From SI Swimsuit 2014."

It's clearly a fake ... but Sports Illustrated's legal team doesn't care -- they fired off a letter to the site threatening to sue if the post isn't taken down, STAT.

In the letter, the lawyer complains, "One of these photographs has even been airbrushed."