Justin Bieber -- Legal Team Threatens Suit Over GIF ... I'm Jacked, Not Puny!

Justin Bieber is angry a website made it look like Calvin Klein pumped up his muscles using Photoshop, and unless he gets an admission he's going to court.

Justin's team fired off a demand letter -- obtained by TMZ -- threatening breatheheavy.com with a defamation case for posting a GIF showing Justin transformed from mildly muscly to jacked.

The Biebs and company say the puny photo is fake ... clearly Photoshopped, and it's damaging his brand.