Loading thumbnails jpg into buttons

Hi i did tried to load into a button an external thumbnail(jpg) into a button with the loadmovie into a target but it’s not center and the rollover actions that i had on my button is not working anymore. i am confused…all i need is to be able to change pictures (thumbs) without changing the FLA and it’s confusing, this is my code
loadMovie(“ad1.jpg”, “btn1”);
it loads but the rollover action doesn’t work and it’s not center in the middle of the button.
Thanks a lot guys
Massimo