How to transistion effect added?

This is my coding, how to create the one picture to another picture transistion effect added?

imgbtn_center.onRelease = function() {
infoField._visible = true;
startLoading(“pic_01.jpg”);
};
imgbtn_up.onRelease = function() {
infoField._visible = true;
startLoading(“pic_02.jpg”);
};
imgbtn_down.onRelease = function() {
infoField._visible = true;
startLoading(“pic_03.jpg”);
};
imgbtn_left.onRelease = function() {
infoField._visible = true;
startLoading(“pic_04.jpg”);
};
imgbtn_right.onRelease = function() {
infoField._visible = true;
startLoading(“pic_01.jpg”);
};
function startLoading(whichImage) {
loadMovie(whichImage, “imageLoader”);
_root.onEnterFrame = function() {
infoLoaded = imageLoader.getBytesLoaded();
infoTotal = imageLoader.getBytesTotal();
percentage = Math.floor(infoLoaded/infoTotal*100);
infoField.text = percentage+"%";
if (percentage>=100) {
delete this.onEnterFrame;
infoField._visible = false;
}
};
}