Lets see how to convert between Bitmap, BitmapData and ByteArray in few examples:

• DisplayObject to BitmapData
• BitmapData to Bitmap
• BitmapData to ByteArray
• URL (image) to ByteArray (asynchronous)
• URL (image) to BitmapData (asynchronous)
• ByteArray to BitmapData (asynchronous)

DisplayObject to BitmapData. FYI DisplayObject is any visible object in flash (Sprite, MovieClip, Bitmap, UIComponent, Video etc…):

import flash.display.BitmapData;
var bitmapData:BitmapData = new BitmapData(button.width, button.height, false, 0xFFFFFF);
bitmapData.draw(button);
// result: bitmapData

BitmapData to Bitmap:

import flash.display.Bitmap;
var bitmap:Bitmap = new Bitmap(bitmapData);
// result: bitmap

BitmapData to ByteArray. Requires jpeg / png or any other encoder. Read more about JPEG encoding here:

import mx.graphics.codec.JPEGEncoder;
import flash.utils.ByteArray;
var encoder:JPEGEncoder = new JPEGEncoder(90);
var byteArray:ByteArray = encoder.encode(bitmapData));
// result: byteArray

URL (image) to ByteArray. Asynchronous operation:

import flash.display.Loader;
import flash.display.LoaderInfo;
import flash.events.Event;
import flash.net.URLRequest;
import flash.utils.ByteArray;

var loader:Loader = new Loader();
loader.load(new URLRequest("banner1.jpg"));
loader.contentLoaderInfo.addEventListener(Event.COMPLETE, loaderComplete);

private function loaderComplete(event:Event):void
{
    var loaderInfo:LoaderInfo = LoaderInfo(event.target);
    var byteArray:ByteArray = loaderInfo.bytes;
    // result: byteArray
}

URL (image) to BitmapData. Asynchronous operation:

import flash.display.Loader;
import flash.display.LoaderInfo;
import flash.events.Event;
import flash.net.URLRequest;
import flash.display.BitmapData;

var loader:Loader = new Loader();
loader.load(new URLRequest("banner1.jpg"));
loader.contentLoaderInfo.addEventListener(Event.COMPLETE, loaderComplete);

private function loaderComplete(event:Event):void
{
    var loaderInfo:LoaderInfo = LoaderInfo(event.target);
    var bitmapData:BitmapData = new BitmapData(loaderInfo.width, loaderInfo.height, false, 0xFFFFFF);
    bitmapData.draw(loaderInfo.loader);
    // result: bitmapData
}

ByteArray to BitmapData. Asynchronous operation:

import flash.display.Loader;
import flash.display.LoaderInfo;
import flash.events.Event;
import flash.display.BitmapData;

var loader:Loader = new Loader();
loader.loadBytes(byteArray);
loader.contentLoaderInfo.addEventListener(Event.COMPLETE, loaderComplete);

private function loaderComplete(event:Event):void
{
    var loaderInfo:LoaderInfo = LoaderInfo(event.target);
    var bitmapData:BitmapData = new BitmapData(loaderInfo.width, loaderInfo.height, false, 0xFFFFFF);
    bitmapData.draw(loaderInfo.loader);
    // result: bitmapData
}

Synchronous JPEG ByteArray to BitmapData can be found here