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Brooke Mueller has been caught with her pants down ... and someone is now hawking the graphic nude photos of Charlie Sheen's ex-wife.

We've learned Brooke took the pics herself, just after she got out of the shower.  The two pictures are really graphic.  The angle is NOT flattering ... and on top of that, she's dripping wet, and not in a good way.  It's kind of like looking at a topographical map of the Rockies.

Brooke sent the pics to her then-boyfriend ... a Jewish country singer.

We're told Brooke knows exactly who's trying to sell the photos -- a disgruntled ex-assistant.