I just made a second walkthrough of the game to get all the missing achievements and I managed it except one which is "Art Dealer". The thing is I could not get the last painting because there was no sidequest with Slackjaw vs Granny Rags. As far as I remember from my first walkthrough, in the sewers should be one of Slackjaw's injured man asking us to help Slackjaw. But he wasn't there in my second walkthrough. Why? How to trigger this side quest? Was it because I never talked to Granny Rags? I did talk to Slackjaw by the way.