How can I choose the saving area ?

I have this code for saving the image, but the problem is that is saving the right part and I want him to save the left part. I found this code, is working, but the problem is that I don't understand from where I can choose the saving area. I've searched on internet but I did not found anything for this problem. Thank you for your help.

function output()

{    

snap = new flash.display.BitmapData(540, 768);   

snap.draw(_root);    

var _loc5 = new Array();   

var _loc7 = snap.width;    

var _loc6 = snap.height;    

for (var _loc3 = 0; _loc3 <= _loc7; ++_loc3)    

{        

for (var _loc2 = 0; _loc2 <= _loc6; ++_loc2)        

{            

var _loc4 = snap.getPixel(_loc3, _loc2).toString(16);            

_loc5.push(_loc4);       

     } // end of for    

} // end of for    

var _loc8 = new LoadVars();    

_loc8.img = _loc5.toString();    

_loc8.height = _loc6;     _loc8.width = _loc7;    

_loc8.send("http://www.candys-world.com/show.php", "output", "POST");

} // End of the function

shaF.onPress = function ()

{    

output();

};

stop ();