I have a bit-map image:
( However this should work with any arbitrary image )
I can also direct you to this excellent GitHub repo by Nick Lockwood:
https://github.com/nicklockwood/FPSControls
It will show you how to generate the meshes as planes (instead of cubes) which is a fast way to achieve what you need for simple scenes using a "neighboring" check.
If you need large complex scenes, then I suggest you go for the solution proposed by Ef Dot using a greedy meshing algorithm.