the painted parts are wrong, dont mix paint+photos
or if you do, do it well.
you need to blur the painted parts
the mountains are off scale and also need to be blured
the building on the left far with the cracks texture is wrong
you need to do that well, re-do it
the road with the crack is completly wrong, cars are also wrong or too sharp or too blured
far buildings in the city (left) are too brights.
the front white building is too clear, and also the paintd parts are too sharp
I feel that, the image lacks a proper focal point too! The mountains look totally detached from the picture; the edges are not rotoed properly and blended too! The best place to start working on a matte painting would be sky replacement and cleaning objects from a picture, then you can think of a cityscape! But, a good attempt anyhow; keep it up! Take care.
I would recommend and suggest you to join an online matte painting course that, I will be organizing at a website/forum from February/March 2017 onwards, which will be taught by a "Shrek 2" matte artist! :) It is geared towards beginner artists like you, who is very enthusiastic! To know more, please e-mail me at : email@example.com. Take care.