you need to use max or blender or whatever and build the objects you see in the image and camera match them. If you are using a standard camera to take your photos then you are most likely using a 35mm lenz.. so make a camera that is that lenz... then you need to put the camera at the correct height.. again if you are taking the shots measure your height and place the camera at the height of where you held it while taking the photos..
Now you have the camera in the correct position... so look at your photo and work out the "
vanishing point' of the photo. Then in your 3D app look though your camera and set the horizon line to the vanishing point in your photo.
Now you can build objects and place them in the photo at the correct perspective. Remember that this is quite hard for beginners but it is really not that difficult.. use real world numeric figures... do when you take your photo try to do some measurements first say of that cube grate thing... how many cmxcm is it... once you have the object you can place it in the camera view correctly.
You can also use objects in the scene to measure.. like for example a brick is 230x78x110 so by counting the bricks in a building you can see how far into the distance it is and how high and stuff like that.
Again this can be hard for beginners but in truth it is not really difficult just a lot of pre-planning when you take your shots.
The question you need to ask is do you need to do all this... once you get the perspective you can just add a huge ground plate for your toon to walk about in and use the wme 2d region stuff to make the impassible objects and crap
Hope this helps and good luck
