If you own megapack 3, you should find a collection named "urban" to get you started. Its quite alright to work with photos once you do this.
Keep in mind that most model pack buildings have a large variety in appearance ranging from almost post apocalyptic to brand new looking. You will need to retexture them to work with your chosen look. A bit of colour correction should do the trick.
Below you will see some city scenes I have made, you can see buildings in the background that are all just simple boxes using a single texture, that way you can create the illusion of a whole city around the player without losing any performance. Its also a good starting point to learn modeling if you haven't already as modern architecture with its cubic look is just a great place to get started.
Notice that if you use concrete sidewalks and roads you can deactivate the terrain and water using the "goinside.lua" script around the start marker (with a zone) and gain a significant FPS boost. Now you can create large cities without worrying about FPS drops.
If you want buildings to light up, duplicate it and ...in your chosen art program... brighten up the windows in the diffuse texture and be sure to add an emissive map.
Here are some examples of cities I made:
Hope this helped and please don't hesitate to post if you have more questions!
Cheers!
-Wolf
