EXCLUSIVE

Lil' Kim is a ghoulish thief who ripped off a creepy pic of a decaying skull ... and is now claiming it as her own ... so says the professional makeup artist who took the pic and is demanding $$$.

Samantha Ravndahl tells TMZ ... she's not only the artist ... she's also the girl in the picture -- which shows a green-faced woman with rotting flesh and exposed parts of her skull.

Ravndahl says she posted the macabre selfie online ... and soon discovered that Lil' Kim was using the very same image on her own social media sites.

Ravndahl tells us ... she contacted Lil' Kim's people asking for payment and got the runaround -- first she was told Kim wasn't willing to pay anything ... and when Ravndahl persisted ... was told that Kim's team "works slowly."

All the while ... Ravndahl says Kim continued to re-post the image on various sites ... even using it as the artwork for her single, "Dead Girl Walking" ... and had the audacity to add a copyright logo.

Ravndahl hasn't filed a lawsuit ... yet .. but she's considering it. She's still hoping to get some kind of financial settlement.