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.  

Bieber's team wants 2 things: remove the post and acknowledge the deception ... or else.