Witch Elf with an Arquebus? ( What is the name of the JPG file? ) ~~~~~// ...did googletrix... //~~~~~ A little bit of Google-fu reveals it is an Escher Ganger from Necromunda, concept art that was used on a promo flyer/poster. Not necessarily available as a sculpted model...convert from the sprues in the boxed set... (if that has been released?)
As @pendrake says it's a concept piece for Necromunda Escher but you never know with GW it could become a model.