I didn't come to that kind of problem yet (meaning never had to attach images that way), but try with BitmapData class and reference as well.
Something like this:
PHP Code:
import flash.display.BitmapData;
var linkageId = "libraryBitmapPaletteColors";//***
var myBitmapData = BitmapData.loadBitmap(linkageId);
var palette = this.createEmptyMovieClip("palette", this.getNextHighestDepth());
palette.attachBitmap(myBitmapData,this.getNextHighestDepth());
*** libraryBitmapPaletteColors is the image's identifier
Hope this helps
Alex