Center image content
If you get a bitmap data as source of an image component and you want to center it you have to create a new bitmap, create a new matrix, apply a transformation (in the snippet the translation is done accordingly to the component with) and then use again the BitmapData class to draw the new data accordingly to the transoformation
var imageData:BitmapData = BitmapData(someRawBitmapData);
var bmp:Bitmap = new Bitmap(imageData);
var matrix:Matrix = new Matrix();
matrix.translate(-((imageData.width - this.width) / 2), -((imageData.height - this.height) / 2));
var matriximage:BitmapData = new BitmapData(bmp.height, bmp.width, false, 0x00000000);
matriximage.draw(bmp, matrix);
// Suppose imgThumbnail is the ID of your component
imgThumbnail.source = new Bitmap(matriximage);