I decided to start with the subway photo. 
Here are my perspective/horizon lines to get started:
It didn’t all line up perfectly, so I had to do some additional tweaks to most of the additions, but it gave me a really good starting point. After I fit the round I voted sticker onto the ceiling, I used the selection tool to change the color so that it would more closely match the lighting in that area. I used layer masks on the Van Gogh on the ceiling and the “Love” graffiti on the floor to try to make the additional images blend better with the subway. Once I got the perspective close, I used the skew tool to make the final adjustments to the place mats (window shades) and other Van Goghs into the spots I wanted them.
I was mostly able to achieve the look I wanted, but struggled a little bit with the ceiling piece. I adjusted the color saturation etc…, but I still think the color quality is off.
