She's referring to the painting being rehung in his apartment, literally to affix the painting ('this') back up to the wall on which he found it.
In the original screenplay the wording is very subtly different, but shows the intention more clearly.
MOLLY stares at the painting blank-faced.
MOLLY: (pause—calmly) That is...an authentic Monet?
HELLY: Grabbed it right off the wall.
MOLLY: (pause) You came over here carrying a Monet.
HELLY: I was driven. And I have security outside the door.
MOLLY: Okay. Helly, here's what I need you to do. I need you to get this, what, 20--?
MOLLY: --27-million dollar painting out of here right now. And your, I assume, armed security--?
MOLLY: --can't stand in front of the door. Fix that up, come on back and I'll extend you credit tonight up to one and a half.