Why can't i change the image of a movie clip easier

seems a little complicated for replacing an image in a movieclip with something else. It seems like a hack actually. Is it really so hard to switch a png (maybe of a different size) in a movieclip with something else? It also appears the livedocs at this time, are down. If it gets any harder I might switch to silverlight if it’s not based off hack(ECMA)script