How to swap the source image of a MC programmatically in AS3?

Question

This seems like it would be a simple thing.
I have converted a jpeg to a movie clip that I want to be able to swap out the image source programmatically in the code behind.
This can be done easily in the GUI. How do you do it programmatically?
I have been searching for hours now and have not found anything that discusses how to do this.
pseudo code:
var my_image = "my_image.jpg";
my_mc.SwapSource = my_image;

Answer 1

Thanks for the reply. Please allow me to elaborate further. I set the class name in the linkage section of the bitmap properties dialog box as you suggested using the format pic1, pic2, pic3....and so on for each of my images in the library. I want to assign these randomly to my movie clips, topImage and botImage respectively.
Following is my code:
// number of images
var myImageSequence = 12;
// assign a rectangular mask to the top image
topImage.mask = mcMask;
btnSubmit.addEventListener(MouseEvent.CLICK, mouseClickButton);
function mouseClickButton(e:Event) {
var myRandomNumber1 = Math.floor(Math.random()*myImageSequence)+1;
var myRandomNumber2 = Math.floor(Math.random()*myImageSequence)+1;
var myImageName1 = "pic" + myRandomNumber1;
var myImageName2 = "pic" + myRandomNumber2;
topImage.addChild (myImageName1);
botImage.addChild (myImageName2);
Following is the error:
TypeError: Error #1034: Type Coercion failed: cannot convert "pic10" to flash.display.DisplayObject.