Positioning of walls

Hello,
I am contacting you because I need help.
Then I is an image in this image I made a script that randomly put walls on the image, the problem is that I want the walls arise randomly in a part of the image and not over the entire image.
So how to say the walls of this position only in this part of the image.
Here is the picture:
http://img812.imageshack.us/img812/3801/fondarene.jpg
So infact it should not go on the walls yellow.

Thank you in advance.