The program::

first of all determine the components of color of the background
of ur image.you can google it to know how you can get color
component of a point.then create a nested loop which
goes over those two photos and replace the selected background
color components,with color components of the tower image's
corresponding pixel.

the code follows-

Bitmap Pixelsreplacement(Bitmap YourImage, Bitmap TowerImage,
Color PlainColr)
{
Bitmap NewImage=YourImage;
for(int j=0;j<NewImage.Height;j++)
for (int k = 0; k < NewImage.Width; k++){
Color Pixel=NewImage.GetPixel(k,j);
if(Pixel==PlainColr){
NewImage.SetPixel(j,i,TowerImage.GetPixel(j,i));}}
return NewImage;}

that is it.you know how to replace the background of a image.it's the
same technique used in the media and movies.