How to i split a bitmap into parts? And use each part for a texture?

You have to unwrap all fifteen cubes in maya and make it share the same texture!! that is the easiest!.. If u want to follow this search for unwraping in google u will get.

If you want to do it in unity u have set the UVs manually by using Material.textureOffset and Material.textureScale

For the sake of others who find this thread.

To divide a square image into 16 parts (4wide by 4high) i did this:

using UnityEngine; using System.Collections;

public class InitCube : MonoBehaviour {

 public GameObject ga;
 float wide; // tiles wide
 float high; // tiles high

 // Use this for initialization
 void Start () {
     float x;
     float y;
     x = ga.transform.position.x;
     y = ga.transform.position.y;
     wide = 4;
     high = 4;
     float scx=-(1/wide);
     float scy=-(1/high);
     float offx=-(wide-1-x)/wide;
     float offy=-(high-1-y)/high;
     ga.renderer.material.SetTextureScale("_MainTex", new Vector2(scx,scy));
     ga.renderer.material.SetTextureOffset("_MainTex", new Vector2(offx, offy));  
 }

}

Notice:

 Think of a brick wall 4 bricks tall by 4 bricks wide.
 Think of one graphic covering all 16 bricks such that the image is intact accross the entire wall.
 Each brick gets a part of the image.
 The GameObject is a cube whose scale is 1,1,1
 The graphic is already a material on the cube surface.
 position x=0,y=0 is the lower left corner of the structure
 position 3,3 is the upper right brick.
 In Unity, the material has a "Tileing" and an "Offset" properties.
 In the c# code the SetTextureScale actually changes the "Tileing" property.

There you have it.