I can deff say that petes bar is the best place for it. Had multiple drops during some waves (2-4 portraits at a time in some cases) I know for a fact that my friends and I deff picked up more than 10 portraits before the acheevo popped. Money getting stolen from the rats was quickly remedied by going in the back of the bar (where some stairs are) and jumping up on a ledge enabling me to stay clear of them.
I'm GMT-05 Eastern(US)
My gamertag is spelled 'LRX7' its not an "i"