Plz help from a beginner

hi, could anyone plz help me out with this, the problem seems to be too simple to find a solution from the web tutorial.

what i want to do is, when certain button is clicked, replace a certain symbol(button) by another symbol(movieclip) stored in the library.

i try things like loadmovie("","") to replace the symbol from a external jpg, but some how it does not align even both images are exact dimension.
can i replace an image straight from the library?